I bought 3 RB67 bodies recently, with backs, and picked up a lens seperately. The bodies all fired perfectly well when the lens wasn't attached, but with the lens on the shutters all seem to stick and won't fully raise. Does anyone have any advice- I'm bemused because I tried the bodies with a different lens (belonging to the man I bought the bodies off) and all of them fired perfectly well with that one. Is it a faulty lens, or me not assembling it correctly?