airtag help, broken?

It's important to understand that the Apple AirTag relies on Bluetooth and not GPS for providing its current location. As such, either your or someone else's iPhone must be near the AirTag in order for it to receive its Bluetooth signal. From there, the iPhone must have either a cellular or Wi-Fi connection to the Internet to send the location information to the Apple servers BEFORE you can see it in the Find My app on your phone.

If I had to guess, while you were at the gym with your phone, no other phone was nearby to pick up the signal.

Tesserax is spot-on.

Airtags work best in areas with good mobile data connections and lots of iPhones. (e.g. Airports)

They perform poorly in areas with few iPhones and/or poor mobile data.

They don’t work at all in areas lacking iPhones or mobile data. (e.g. deep rural or wilderness areas)

Also be aware that an AirTag doesn’t transmit it’s beacon continuously. (that would require too much energy)

Anecdotal evidence indicates beacon transmits occur maybe every 10m or so.

Then, in theory, if each of those other iPhones had "Find My Network" enabled on them, your AirTag should appear in the Find My app on your phone when you were away from it.

There is also a possibility that this tag is defective ... and should that be the case, and you got it within the last 14 days, you can return it for either a replacement or a full refund. After that, you can still bring it to your local Apple Store or Apple Authorized Reseller to have them verify that it is indeed defective and you should be able to get a replacement at little or no cost to you.

Alternately, you can 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.