charlie902,
This is expected for the following reasons which are mentioned in Find your keys, backpack, and more with AirTag and Find My - Apple Support. The AirTag will need to be in range so will only appear on the iPhone that is within range.
"Use Precision Finding to pinpoint the location of your item with Find Nearby
If your AirTag is within Bluetooth range, tap Find Nearby to help find it, even if it's stuck between couch cushions or hiding under a stack of mail."
"Find a nearby item
- Open Find My, then tap Items.
- Choose your AirTag, then tap Find Nearby.
- Follow the onscreen instructions and move around the space until your iPhone connects to your AirTag.
- Follow the distance and direction information that your iPhone displays. Your iPhone also vibrates as you get closer to the AirTag.
- You can tap Play Sound to hear a series of beeps that helps you find your AirTag.
- If your iPhone says that more light is required, uncover the camera or tap the Turn Flashlight On button.
If the Find Nearby button doesn't appear
If you don't see Find Nearby, make sure that you turned on Location Access for Find My.
- Go to Settings > Privacy & Security > Location Services and check that Location Services is on.
- Scroll down and tap Find My.
- Choose how you want Find My to access your location.
- For the most accurate location, turn on Precise Location.
Still don't see Find Nearby? Your AirTag might be out of range of your iPhone — first try finding it on a map and getting directions to its location."
All the best.