Sounds like a couple of possibilities.
1. You don't have enough antenna and power to hit the repeater, even though you can hear the repeater. This happens when a repeater is up higher than you, uses more output power than you, and has a better antenna system than you.
2. Perhaps you do not have the radio programmed correctly? TX tone is crucial to hitting the repeater and getting a response. Verify you have the correct TX tone and for troubleshooting purposes remove the RX tone if you have one set.
What radio do you have, that might help us, help you.