Depending on different aspects I would check:
- Are you transmitting at all? Maybe reach out for someone and ask for a simplex contact
- Check if you have enough signal strength (e.g. by trying to work the repeater in direct line of sight)
- Check the resonance of your antenna (esp. if it is 3rd party)
- Check if you have the correct subtone active.

What is the repeater call? That might indicate other topics as well. Maybe reach out to the sysop and ask.