B4.1 Rear wheels binding

[Callum]

Hey chaps,
Was running my B4.1 last week at my first 10th race all was going well until the final were I had only one wheel driving on the back on further inspection the wheel was getting jammed with grass this only happened when the track became wet, there is only about 2mm between the hub and wheel hardly any clearance at all.
Just wondering how I can prevent this from happening again........
Thanks Callum