AirTag lost connection

There could be a number of reasons, but without more information from you, it will be hard to suggest what they may be.

For example:

• Is this a brand new AirTag?
• Was it set up properly initially? Ref: Add an AirTag in Find My on iPhone - Apple Support
• Which exact model is your iPhone and what version of iOS is it running?
• Have you tried resetting the AirTag? Ref: How to reset your AirTag - Apple Support

So, again, just to make sure I understand. You had successfully paired this AirTag with your phone. It was near your phone for at least 12 hours, then disappeared from the Find My app.

You then decided to try setting it up again; it paired successfully, but again the AirTag no longer shows up in the Find My app after a certain period of time ... correct?

If so, then it may be possible that either the battery (that came with it) or the tag itself, is defective. As such, I would suggest that you contact Apple Support directly for further assistance with this matter. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.