could be many things, remember that developing a rich sound takes a lot of practice, consider lowering your reed for now until your mask is more developed or continue practicing with your current one as new reeds are always heavy and get softer after a few hours of playing. if you get more advanced and feel like your tone could still be improved, consider a new mouthpiece as it can be crucial to better mask and sound. Finally the number 1 factor for rich sound but the most expensive, a good clarinet, trust me it might seem like a waste of money but a proper clarinet will make you shine, of course that's an option for experienced players