Type 1 Injection site

If you're regularly injecting into the same area you could have Lipohypertrophy in that area where fatty tissue is forming from overuse, this can sometimes lead to slower absorption.

It could also be that you just didn't inject enough (especially if you didn't do an air shot beforehand to make sure insulin was coming out of the pen), bad insulin, faulty needle etc. If it's a one off I wouldn't worry too much but just make sure you are rotating your injection sites often.