** [Maytag washer] model mav3855aww doesn't always spin the clothes very dry **
The belt could be slipping too much.
Is the washer draining properly? Too much water in the tub will prevent it from spinning properly and could eventually damage the belt leading to more problems.
** sometimes it doesn't go into the spin cycle at at all **
What is it doing instead at those times? Any sounds? Is the motor running? Does the timer advance to the end of the cycle anyway?