They don't show up as "can't send" but the other person never gets them. This has been happening for a while and I'm missing a lot
Is it just the one person that's not getting them? Or is it multiple people? Which carriers do they have?
Have you restarted your phone lately? Shut it down completely and wait a couple minutes, then reboot normally. This can sort out a variety of issues, especially network connections.
If you previously had an iphone, it's probably apple's fault. Follow the instructions here Issues with texts after moving to Android from iPhone? Here's an easy fix! | Android Central