Sometimes insulin loses some effectiveness due to inappropriate handling. Insulin should not be shaken, even though that is very tempting. The bottle should be gently rolled between the palms until the insulin is mixed in solution well again. The site of the insulin injection may need to be changed in order to allow consistent absorption. Usually it is pretty consistent if given on the sides of the trunk somewhere.
Many dogs require twice daily insulin dosages to regulate diabetes.
Some dogs need twice daily dosing and mixtures of different types of insulin. Usually when things get this complicated we refer our cases to a internal medicine specialist or veterinary school.