There should be a pressure balancing spool of some kind inside the valve and it is sticking, or broken. Remove the large plate and see if there is a "nut" of some kind on the front of the valve, usually at the lower right, but maybe on the left. IF so, that has to be removed and a new one inserted. BUT, you have to purchase the one for that brand of shower valve. If there is not that piece, then the spool is probably integral with the control unit and you have to replace that part.