Replace all of your grass with golf course bermuda and cut it really short.

It's just the nature of the mechanics that grass will give these tanks a fit. Too many things for the tracks to grab onto and most of the time grass is of a thickness which creates great resistance against something that small trying to maneuver in it.

Most RC vehicles I own have a rougher time in grass than anything else. This goes for my Brushless LST2, Brushless Revo, and Axial SCX10.