Assuming you have the owner's permission to access the repeater or it is an Open repeater...
Channel 15 "should" be the corresponding "radio to radio" channel. If your radios are able to talk to each other on channel 15 then make sure the "code" is set to none or off and listen for a while. You can switch back to repeater channel 1 once you know the radios are working fine on channel 15, just make sure the receive code on repeater channel 1 is set to none or off as well.
If you still hear no activity then either the repeater is inactive at the moment or you are out of range. Range is affected by more than just the owner's estimated radius. Buildings, hills, trees, and other things can interfere with radio signals.
Try moving close to the repeater and (with transmit code set to match the repeater's input code) transmit your FCC GMRS ID . Listen for a "kerchunk" or short burst of static as a confirmation that you have "connected to the repeater". Keep moving closer to the repeater and trying again until you get a reply (either a burst of static or another person).
If you still can't connect to the repeater you may need to contact the owner to see if the repeater is even online. But if you do connect...
Once you have a confirmation you are reaching the repeater, start moving further from the repeater until you get back to your original location. Note if and/or where you lose connection.
You could try a better antenna and/or try different locations. (I have a sweet spot at home where I can reach a repeater 20 miles away, but if I move a couple inches I loose the repeater completely.)
The process is a lot easier if you have two people working together on this with one of you staying within connection range of the repeater while the other moves around. (Create your own traffic.)
Unfortunately it is possible the repeater is simply not reachable and there is absolutely nothing wrong with your setup. Keep checking from time to time and from different locations of you're sure the repeater is online.
Jim