Self Propelled Lawn Mower Not Moving Forward on Grass

I have a Briggs & Stratton High Wheel Variable Speed Gas Walk Behind Self Propelled Lawn Mower with Bagger ( model#21378) that was purchased 8 months ago. I was cutting the grass a couple weeks ago, and the mower stop moving forward half way through cutting. My neighbor has a passion fruit tree on their yard; the mower got stuck on a branch. I pulled it back a couple inches and move around the branch. That's when I realized the self propelled no longer working when engaged on the grass. I pushed the mower on the driveway and engaged the self propelled again, and it worked there. It stopped working on the grass but worked fine on concrete.

I did a little bit of troubleshooting. I removed the wheels to check for debris; they were cleaned. The driving belt was on the pulley. The belt did not look damage. When I engaged the self propelled, I noticed the belt tighten up a little. However, the cable felt a little loose. Do I need to replace the cable, or could it be something simpler?

The mower is still under the company's warranty, but the approved dealers told me they are 3 months behind and would not be able to get to it before that. I am hopping to find a solution online and not having to take it to the dealer.