Small Engines (Lawn Mowers, etc.): mower wont move, belt tension, tension spring

Question
I have an MTD walk behind self-propelled mower model 12A-469B000. When I engage the front wheels on grass the mower won't move. If I engage the wheels on concrete, the mower moves. I have replaced the drive belt, but no luck. There is no tension pulley to adjust and the belt tension spring is maxed out. Any ideas?

Answer
Dan,

   If you can try to adjust the front wheels down like you can do with  regular Mower when on grass there are depressions because grass is not a solid level area like concrete what your wanting to do is put more pressure on the front end to give it something to ride on thus shifting the weight forward.
A trick one of my patrons did with his MTD was he placed small 2 pound sand bags and iron weights on his front end and this allwed more weight for the driveline to grip and go when he finally got the desired weight he went to pure sandbags

Another cause could be the pressure gear is not engaging the weight trick comes in to play on this