Honda izy self propelled not pulling very well?

my Honda Izy mower was working perfectly well until I went over a pile of very dusty grass . It now seems that the wheels don't pull as well before it would pull itself very well even on inclines but now it's almost not self propelled. When I lift the wheels off the ground they are still turning but have no power.could the gears be full of rubbish if so how could I clean them out . Thanks