What makes my mtd lawn mower will not change into higher gears it moves really slow in 7th gear

My lowe's 42" cut riding lawn mowers want change into the higher gears and i have to fight it to get it to go into reverse and forward, what causes this?

I would say you need to replace your drive belts , or it could be the variater drive is not moving up and down same with the Lowe's mower.
Luck