If it was properly repaired and cleaned recently then it may just need some exercise. Mechanical shutters like to be used regularly.

The last thing you'd want to do is put any kind of lubricant on the shutter blades. The cleaner and dryer they are, the better. The only parts that sometimes need a minuscule bit of lubrication are a couple of the escapement gears.

Trying repeatedly firing the shutter and see if it improves any. If not, or if you can see obvious oil or dirt on the shutter blades, a full disassembly and cleaning is recommended.