Text Messages are disappearing after one text

Hi SailorBill,

Thank you for posting to Apple Support Communities.

We’ll need some additional information to provide more specific support.

You mentioned you both are texting to and from your Apple ID email address. Do you share the same Apple ID / iCloud email? If so, this may cause some issues with sending and receiving messages to each other. To resolve this, one of you should create a new Apple. This process is explained below:

1. "Open the App Store and tap the sign-in button .
2. Tap Create New Apple ID. If you don't see this option, make sure that you're signed out of iCloud.

3. Follow the onscreen steps to provide an email address, create a strong password, and set your device region. The email address that you provide will be your new Apple ID.*

4. Enter your credit card and billing information, then tap Next. You can also choose None. Learn what to do if None isn't showing or you can't select it. You won't be charged until you make a purchase. 

5. Confirm your phone number. This can help to verify your identity and recover your account if needed. Tap Next.

6. Check your email for a verification email from Apple and verify your email address.

After you verify your email address, you can use your Apple ID to sign in to the iTunes Store, App Store, and other Apple services such as iCloud."

This information is from How to create a new Apple ID - Apple Support.

Has your girlfriend powered off the iPhone and restarted? Sometimes this can help resolve these types of issues with message conversations. To complete this step, please follow this:

"1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.

3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo."

Restart your iPhone - Apple Support

Take care.

Hi SailorBill,

To address this behavior, go to Settings > Messages and turn Messages off and back on. Have your girlfriend delete your contact card from Contacts and add it back. If it hasn't been done, update iPhone to the latest software to ensure performance enhancements have been added.

Delete a contact

When you delete a contact, it permanently deletes it from your device. If you delete a contact from your email account, you also permanently delete the contact from all devices signed into that email account.

1. To delete a contact:

Open Contacts and tap the contact that you want to delete.

2. Tap Edit.

3. Scroll Down and tap Delete Contact then tap Delete Contact again to confirm.

Find these and additional details in Manage and delete contacts on your iPhone or iPad.

Contact Apple Support if this behavior is ongoing.

Contact - Official Apple Support

Regards.