If it shifts easy when not running but hard to and grinds while running, like doug80638 says, the belt has enough contact with the primary to spin the secondary and thus the transmission. With a CFM dry-clutch system many times it's one of two reasons. The first and most often reason is that the operator allows the unit to idle for long periods which excelerates the wear of the one-way idler bearing the belt rides on while idling and seized or running dry it will make the belt spin. The second most often reason is the primary can't or isn't releasing enough to withdraw sheave contact with the belt. Usually due to some interference in the flyweight area...like packed with dust.
As a rule, belts wear thinner and stretch over time which would make belt wear less likely to be the cause of your problem..unless it's coming apart... so...apart it must come for an inspection. I'm sure the cause will present itself.