This is a common problem with self-propelled lawn mowers. Over time the mower does not want to go in reverse and the wheels lock up and you must drag it in reverse.
Let’s think about the environment that a mower operates in for a minute. It is a dirty and dusty one and this dirt and dust permeates everything. The dust will enter into the rear drive axle of a lawn mower and begin to build up inside it. This dirt becomes a grinding paste that wears on the plastic inserts in the wheels. It will also build up inside the spring loaded key that is located on the axle and prevent it from moving.
So as the dirt and dust build up, it freezes the mechanisms that allow the wheels to move in reverse and that is often the issue.
You can pull the wheels off the mower and see if you can clean the axle and drive mechanism to see if it helps, but most of the time you have to remove the axle assembly and replace parts.
If you need a self-propelled lawn mower repair shop in your area, visit our locations page for assistance.