I've been Exactly where you are now.
Its binding and the gearbox is worn.
If there is no binding, both sides raise with very little rocking.
It has to be extremely square from all axis, and completely clean with syl-glide added for lubrication.
It took me several hours to understand the many ways it can bind slightly.
It will always work, but it will be rocky.
No washers needed to fix it.
Just take time to square it up from all directions.
If its low add square shims or even strips of vellum (it doesn't compress)
Washers sink into the wood real fast.
I replaced the motor/gearbox assembly, then proceeded to work out the binding.