Hello Alex,
Thanks for contributing to the Apple Support Communities.
It seems like after sending text messages to some contacts with iPhones, you're not able to send them iMessages. It is frustrating when features on a device stop working as they should. There are some good troubleshooting steps that can help you send iMessages again.
First, try toggling iMessage off and on again:
Go to Settings > Messages and turn iMessage off, then back on.
If you still can't send iMessages to your contacts with iPhones, try signing out and back in to iMessage:
- Go to Settings > Messages > Send & Receive, tap your Apple ID, then tap Sign Out.
- Go to Settings > FaceTime, tap your Apple ID, then tap Sign Out.
- On your iPhone first, sign back in to iMessage and Facetime.
- Sign back in to iMessage and Facetime on your other devices.
You can find these steps and some additional troubleshooting help at this link:
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
Best Regards,
Jeremy