By not knowing exactly what model you have would make it tough to say. But You can check these things.
If there's a level in the back that you have to pull out, to push it while it's in gear, (automatics), that lever need to be released inward. As it works on a spring.
If you have a loose drive belt.
Some models, Yard Machine, have a lever on the dash board, that uses the brakes in increments.
You're brakes could also be grabbing.