Freezing  destroys the molecules of ANY insulin; any that has either been frozen or is suspected of having been frozen should not be used. Insulin which has been frozen will not be able to do an effective job of controlling blood glucose. 
- Check for discoloration or floating objects in the insulin -- it may also be contaminated.
Insulin can become contaminated by reusing syringes. Syringe needles have a silicon coating designed to make injections less painful. Re-use not only blunts the needle, it also wears away the protective silicon coating; white precipitates  can form in the vial from the silicon, possibly interfering with the action of the insulin. 
It's also possible that the new or newer vial from the pharmacy may be flawed. If you've recently started it and are having problems, this might be the case. Taking down the lot number and getting a new vial that has a different batch/lot number should take care of this.
Frosted insulin: If insulin is subjected to temperature extremes, such as freezing or overheating, the insulin can precipitate  on the vial's walls, giving it a frosty or frosted appearance.  Another term used to describe this is flocculation.  In the photo above, the insulin vial on the right is a visual example of what a frosted vial would look like. You can see the precipitated insulin clinging to the sides of it.
If you made a mistake and forgot to put the insulin back into the refrigerator, even for several hours, there should be no problem. Many keep their insulin (see instructions for your brand) at room temperature all the time . When comparing it to insulin which is in the fridge except when used, it may be more likely to have slight potency loss. When a vial has been left out of the refrigerator for some time, it needs to be monitored for any signs of frosting;  in the photo, the vial on the left is an example of frosting. If the insulin was exposed to heat or direct light for a while when it was out of the refrigerator, or shaken vigorously or dropped a long way, the best thing to do would be to start with a new vial. 
Don't be in a hurry to discard the "old" vial of insulin you were using. If your insulin is one that needs to be ordered, the old vial is all you have until the order comes in. It can be potent enough to see you through until you get your new one; undamaged insulin loses potency gradually.  If there's nothing wrong with the old vial except that it's begun to lose potency, don't be afraid to go back to it if something seems not right about your new vial.  Keeping your old vial until you're certain all's going well with using the new one is a good idea.
Bad/Weak Insulin: The US FFDC Act of 1941 required anyone producing insulin to certify the "purity, strength, quality and identity" of each batch of insulin.  Since the repeal of this act, no insulin sold in the US has been required to undergo batch testing since 1998.  As per US Federal Register: "With the repeal of section 506 of the act, and the elimination of the insulin batch certification program, the agency is eliminating these subparts." This means that none of them--whether they are genetically produced or the newer analog insulins, need to do this to be sold. Older or newer, any vial of them CAN be substandard. 
Getting a different vial from a different lot/batch number can tell you quite a bit about whether there's a problem with the insulin or the pet.
Before each use, take a moment to inspect the insulin prior to drawing it into the syringe; clear insulins should appear not discolored and clear; suspended insulins should be uniform in their cloudiness. 
Do NOT use the insulin if:
- ↑ Insulin FAQs. Pet-Diabetes.co.uk.
- ↑ Freezing Insulin; Injection Changes. Diabetes Forecast (August 2008).
- ↑ 3.0 3.1 Precipitation (chemistry). Wikipedia.
- ↑ 4.0 4.1 Glossary. Sugardiabetes.net.
- ↑ Vetsulin-Technical Bulletin-Page 3. Intervet.
- ↑ Dowling, P. M. (1995). Insulin therapy in dogs and cats-. Canadian Veterinary Journal.
- ↑ 7.0 7.1 7.2 7.3 7.4 Rosskamp, Ralf H. Park, Glen (1998). Long Acting Analog Insulins-Frosting Caused in NPH/Isophane Insulin By Heat/Cold. Journal-Diabetes.org.
- ↑ 8.0 8.1 8.2 Flocculation Definition/Description. Dorlands Medical Dictionary.
- ↑ 9.0 9.1 Patchuki, Linda, et. al. (1 November 1987). Flocculation of Insulin Preparations. Annals of Internal Medicine.
- ↑ 10.0 10.1 10.2 Playán J, Acha J, Navarro H, Sanz A, Guallar AM, Albero R. (1994). Flocculation of NPH Insulin. Revista Clinica Espanola-(English Translation).
- ↑ 11.0 11.1 Insulin Update-Storage of Insulin. Nora Apothecary.
- ↑ 12.0 12.1 Travelling With Insulin. FAQs.org.
- ↑ Tips on Caring for Diabetic Pets. Diabetic-help.com.
- ↑ Ask the D Team. Children With Diabetes.
- ↑ Human Guinea Pigs-How to Tell if Your Insulin Still Works. Diabetes Mine.
- ↑ 16.0 16.1 Humalog & Heat. Diabetesnet.com.
- ↑ Humalog & Novolog. Diabetesnet.com.
- ↑ Insulin Batch Laws-page 50. US Autism 2009 Regional Conference.
- ↑ Repeal of Insulin Batch Testing Law. Federal Register (1998).
- ↑ Prescription Drug User Fee Reauthorization And Drug Regulatory Modernization Act Of 1997-Section 23. Thomas.gov.
- ↑ 21.0 21.1 Benson EA, Benson JW Jr, Fredlund PN, Mecklenburg RS, Metz R. (1988). Flocculation & Loss of Potency of Human NPH Insulin. Diabetes Care.
- ↑ Insulin Therapy-Stability & Storage. RxEd.org.
- ↑ Keeping an Eye on Your Insulin. Diabetes Health (2001).
- ↑ 24.0 24.1 Diabetes Forecast-Storage & Safety-Frosting of NPH, Lente, Ultralente Insulins-Page 5. American Diabetes Association (2006).
- ↑ Injecting Insulin-Transcript of American Diabetes Association Videotape. American Diabetes Association (2003).