You might not be sending the tone needed to open the repeater, or
You might be transmitting on one of the simplex channels rather than the corresponding repeater channel, or
You’re transmitting with 5 watts and a rubber duck antenna from your driveway and transmissions of the people you are hearing are being retransmitted at 50 watts by a repeater from a high gain antenna that’s 300 feet in the air, or
All of the above.