Perhaps the clutch plate center section that slides on the input shaft is a little rusty, or the input shaft itself could be a little rusty. I had one like this once and it didn't always release correctly. It was also worn out anyway so I just replaced it but maybe you could spray some kind of lube or something ( wd40 or liquid wrench or ???? ) onto the shaft from the dust cover panel on the front of the bell housing. Be real careful not to get any on the cluth lining or friction surface.