If its of the older type it will have a cavity and when you turn it on you will hear it 'singing' to you , a high pitched sort of noise.
Other things that may be wrong is the framing is out or its low on transmission power. Check your antenna for damage and corrosion
Some of the older units and encoders need a warm up time . Don't just turn it on from cold when ATC requests it. Warm up time can be up to 8 minutes.
Really you need someone to go over the system with a transponder test box. Mostly the systems are simple and easy to fault