721D Power in left side weak

I have a 721D that the organization bought this spring. The left side drive seems weak going forward, but has power going reverse. I checked the belts as advised in another post. Both belts seem OK. The Mower has around 1500hrs on it. Don't really know any history on it. I don't have alloy of experience with the zero turn mowers, but I don't think it seems right. Thanks