Old messages, especially those with video or photos, can slow down iMessage. Check the tab for whichever ones you want to use. You should see every device you have registered to your Apple ID. Go to Settings > Messages > Text Message Forwarding.

If you want to be able to send and receive iMessages on another Apple device, start on your phone, or whatever device has the SIM card attached to your phone number.

This was an issue in iOS in early 2018, where iMessages for a given conversation would show up in different sequences for the people in the conversation, like a movie script that got jumbled up. If you don't see anything, move around until you can get a signal, or find nearby Wi-Fi. One way to check if you have access to data is to do a Safari search or refresh your Instagram feed. Your phone will likely resend the text as a green bubble SMS. What this means is that if you've exceeded your phone plan's data limit, or you're out of range of LTE or Wi-Fi, iMessage won't work. Green texts, on the other hand, move text around the world over a basic cellular signal, like a flip phone of yore. The issue is that iMessage uses data, the same type of information-dense signals that feed your YouTube and Instagram apps. They might not have service, or might have their phone in Airplane Mode. If you know the people you're texting are on Apple devices, but you're still getting green texts, then a couple of things could be afoot.

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