450c question

I have an older 450c don't use it much, just a toy. But my question I tried to use it yesterday and was trying to push a hole to bury some brush. When I got the hole deep enough I had trouble getting the 450 to climb out of the hole the right track to pull. The left one seemed to work fine but when I would get almost out of the hole the right track would stop like I had pulled the brake. On level ground it seemed to roll fine but when put in a hard pull it would stop. Any one have any suggestions on what is starting to happen? And what can I do to fix it. Thanks in advance