Also the key here may be that you switched carriers. Who did you switch to/from? How long ago? Some carriers may not support the 'chat' (RCS) features, may take some time to register your number with the service (kinda like switching from iPhone to Android), and in some cases, you may need to re-start the 'chat' with the number you were texting as these conversations are encrypted and may be linked to your old carrier as well. (RCS isn't supported yet across all carriers/devices and it won't work if you're texting an iPhone, just FYI).