We do not know, since there are countless possibilities, so without elaborating closer on the problem, or explaing what you tried to connect the device with the phone, and telling at least what model of Garmin device you own, and what telephone model and Andorid version you use, we cannot even try guessing what may be wrong in your case.
However, you can follow the tips in theSync / Connection Troubleshooting Guide- it covers all known possible reasons of connectivity problems, and offers solutions for them.
