I have two riding mowers that are doing the same thing. When you put them into 1st or 2nd gear they think about moving but the belt just doesn't seem to grab. Once you get into 3rd and above it works fine. Is there typically an adjustment on the drive belt systems for riding mowers to take up a little more slack in the belt? Or, are these belts just worn bad enough that they need to be replaced?

Hi,
Too little information! Brand/model needed to answer. What you describe is the reverse of a wear problem, generally.
Peace,
crigby

Tractors all have adjustments to the tensioner pulley arm. When you depress and release the clutch peddle the arm swings out to apply or relieve the tension, adjust that arm by turning the nut of the threaded rod. If you are out near the end of the rod you may be ready for new belts, always buy OEM belts if you can.