Some very good suggestions already given. Additional suggestions:
-it is ideal if you can replicate the issue (it happening randomly are the toughest to resolve)
-Try a reboot. (a Force Restart)
-Are you getting uncomfortably close to running out of storage space?
-Any apps installed or settings changes very recently?
-happening with specific people you’re texting or is it happening to anyone (if only specific people then it points to carrier)
-If these are iPhone users you are texting, can you connect to Wi-Fi just long enough for testing, to see if iMessage/now blue bubble destinations doesn’t have the issue (if blue bubbles don’t have the issue, it points to signal/carrier issue).
- Either way, I’d open a support ticket with the carrier. It could be happening to a certain small subgroup of their users. It may just need a ‘reset’ on their side. Plus users opening support tickets keeps people like me gainfully employed