Hi. My name is*****’m a camera repair technician with over 20 years in the business and I'll be glad to help you with your question today.
Several things can cause the lens not to stop down properly. Oil or some other contaminent on the diaphram blades, damaged or misaligned linkage, a weak or missing spring or it may be out of calibration.
Any one of these will cause the problem you describe. This really isn’t a user serviceable problem and should be repaired by a qualified technician. You may want to check locally for a camera repair shop in your area, or, if you need assistance finding a shop, please let me know and I'll locate one for you.