What Causes The Blood Sugar Level To Stay High After Insulin Injection?

I am a caregiver for an 87 year old man with type 2 diabetes. we were instructed to give an insulin shot of 8 units if his blood sugar was between 350 and 400. we did inject the 8 units of humulin R and 30 t0 40 minutes later, we took another test and it was 406. Why?