Well let's start with the simple things.
Have you cleaned up the floor board and inspected around the mountings to make sure there is no mud, stones or other debris that would limit the travel of the controls? A little lubrication might be in order.
Have you followed the linkage from point to point to make sure it it not bent or contacting part of the machine or other component?
Next would be to verify the adjustment of the linkages.
Then check the hydraulic hoses for the entire circuit, for kinks, or crushed or kinked hoses and tubes, areas of deterioration. Visible Leaks?
Make sure and change the hydraulic filter.
Next in line would be a pressure test on the drive motors. You'll need the manual and some specialized tools for that.
I am sure I may have left out a few things, but this will get you started.