Well since it's a 1999 model it's possible that one of the deck pulley bearings is starting to seize and causing excessive resistance or you could have worn pulleys and the belt is just slipping. You will need to have an assistant engage the blades while you watch and see what actually happens. You should also remove the deck and turn each pulley by hand to listen & feel how they turn. Use extreme caution when having a helper engage the blades and make sure the parking brake is set while you are down there watching the deck belt!
The problem could also be worn linkage in the engagement controls. There are lots of possibilities and you will need to check the above items and report back with your findings. Of course it could be something other than mentioned but you are the eyes and hands in the situation and will need to tell us more information after you check it further.
