This problem is hard to fix . You have to remove each blade of the diaphragm and then remove any "grease" dirt from them with "gasoline" for lighters (or "dry" solvents) and inspect the blades to see if they are not oxyded on their edges ! Then , you have to polish the blades with polishing media , then clean one more time with dry solvent and if your polishing give a too "blank" aspect , you have to "reblue" the steel with a gun blueing chemistry.
I did the whole thing on a czj tessar's diaphragm, only for pleasure's sake ..... A near invisible oxydation of blades is sufficient to prevent the perfect "slipping" of blades on themselves. I have a K mount Soligor in this condition ... If your lens suffers only of old dry grease on blades , cleaning would be enough. The "old dry grease" appears on elements and diaph blades after an undesired sun/heat exposition . The grease of helicoïd evaporate and condense on the nearest surface ... The "light" fix is to remove the optical groups and rinse the whole diaph mechanism with plastic non corrosive solvent (dry fuel or gasoline is the best - known under the names of "essence C or essence F" in France. ) - Avoid the use of trichlorethylen or aceton if you're not sure the lens contains no plastic parts ... Then you exercise the diaph mechanism until it dries. I did so with a russian Apo-Zenitar and started the job again a few days later and unmounted completely the mechanism. This work is sometimes perilous with some lenses...