A damaged or loose walking belt will slip on the front roller. A bad bearing on a roller can cause the roller to bind and the walking belt to slip.
Check both rollers to make sure that they spin freely. If a roller binds up because its bearing is bad, replace the roller.
Check the walking belt for tears and replace the belt if torn or damaged.
Check walking belt tension by lifting the middle edge of each side of the belt. You should be able to lift the belt's edge up 2-3 inches above the walking board. Tighten belt tension if you can lift the edges of the walking belt more than 3 inches.
These repairs may help solve your treadmill problem
Replace the treadmill walking belt
The walking belt rotates rubs over the platform while walking which can wear and cause the walking belt to slow down when walking. Replace the walking belt when worn or damaged.