You have been using the clutch to hold it on steep slopes, rather than the brake? It sounds as if you have glazed the forward clutch, or just possibly even worn it out. (Of course it is also possible that someone has unwisely oiled the square shaft the clutch plates slide on, and it has thrown out centrifugally onto the forward plate.) You can find out whether the clutch is slipping fairly easily. Start the machine from cold and drive it with full pressure on the drive pedal, up a slope that causes the mower to squeal. (Do not do any manoeuvring of the mower, and especially no holding it on the clutch, during this test). After it has been squealing for a while, stop the mower, stop the engine, disconnect the spark plug leads, and touch the forward clutch disk. Then touch the reverse clutch disk. (Be careful, start with a light quick touch of a fingertip until you see if it is hot enough to burn you.) If the forward one is hot and the reverse one is not, the forward clutch was slipping when you drove up the slope. We can then discuss what you can do about it.
