I am only receiving SOME of my iMessages

I thought some of my friends were ignoring me or only choosing to answer certain texts of mine. Only later finding out that iMessage is either delaying CERTAIN messages from coming through or i am not receiving them at all. This is really frustrating.

When I log on to my messages app on my imac i will discover texts that did not come through to my phone.... this has never happened before and I am really confused as to why this happening and how I can fix this. Thank you so much!