It is likely that air is bypassing the vanes in the motor. This can happen if one or several motor vanes are stuck in place.

If the compressed air is contaminated with dirt or water, it can cause a sticky residue to form and this can cause the vanes to stick.

While it is important to keep your tool well lubricated, see When and how to oil a pneumatic sander, using improper and heavy oil that is not designed for air tools can also cause vanes to stick.

Only use a correct type of oil meant to be used in pneumatic tools.