One of my testicles is not producing semen

When I ejaculate, I feel normal but when I check my testicles for cancers, I find that one is still full of sperm and the other is empty. I tired masterbating more often to see if they would even out but it hasn't. I think only one of my testicles if giving off semen and I don't know why. On the one testicle that has been ejacularing sperm, there are a couple is cysts that I had checked out. My doctor said they were normal and I shouldn't worry. I have an ultra scan and everything so I am pretty sure that is ok, but I don't know if it is related to my problem with the sperm. I don't know too much about this stuff and am hoping that this may be completely normal. If not I would like to know how urgent it is to get this checked out.