Which of the following could be the reason you are unable to access a repeater whose output you can hear?
Why Hearing a repeater's output only proves your receiver is on the right frequency; transmitting into it requires three more things to line up. Your transmit offset (usually 600 kHz on 2 meters, 5 MHz on 70 cm) must be the correct size and direction, and if the repeater is tone protected your CTCSS subaudible tone or DCS code must match what the controller expects. Any one of these being wrong lets you hear the machine but never key it up, so all of the listed causes are valid.
Watch out It is tempting to blame only the offset, since that is the most common setup error, but a perfectly correct offset still gets you nowhere if the repeater requires a tone or code you are not sending.
Hearing is receive; accessing is transmit. Check offset, tone, and code before blaming the repeater.