Some incoming messages (not all) are not transferring from my iPhone 13 to my iPad. This has just started happening.

Hello ethel97,

Welcome to Apple Support Communities. 

We understand that you are not receiving all messages on your iPad. We are happy to help.

We see that you have come to us from an article about transferring your content to your new iPad. Are you using the same Apple ID on both your iPhone and iPad? Are you saying that not all of your content transferred from your iPhone to your new iPad? We recommend that you try the steps here if you are not receiving messages on your iPad as expected: If you can't send or receive messages on your iPhone or iPad - Apple Support

"If you have issues with Messages after you set up a new device

If you experience issues like conversations in Messages showing up as separate threads or sent messages appearing as green message bubbles instead of blue message bubbles when you set up a new device, update your settings using the following steps:

1. Update your device to the latest version of iOS or iPadOS if needed.
2. In the Settings app, tap Cellular. Make sure that your phone line is turned on. If you use multiple SIMs, make sure that the phone number you want to use is selected and turned on.
3. In the Settings app, tap Messages. Turn iMessage off and then back on.
4. Tap Send & Receive.
5. Tap the phone number that you want to use with Messages.

If you have issues when you try to receive a FaceTime call after you set up a new device, you can also update your FaceTime settings."

Let us know if you have further questions.

Take care.

Hi ethel97,

Regarding your iPhone being the new device, the steps that we provided would still be appropriate for troubleshooting this issue.

Additionally, do you have Messages in iCloud enabled on both of your devices? It's possible that having this setting disabled on one of your devices may be the cause for all of your messages not syncing as expected:

Set up iCloud for Messages on all your devices - Apple Support

Cheers!