Sometimes people don't get my iMessages but it says delivered?

Recently, I have thought people have been ignoring me because they never responded. But when I ask them why they didn't respond they always say they didn't get it and they thought I was ignoring them. Basically my question is how I can fix this issue about how my iMessage will stop doing this. It's getting really frustrating. Is there a solution? And where they will be able to get my texts I sent before? if so please respond, i am in desperate need of help for this it's starting to cause problems with work.