One minor thought, depends on how old the fan is. If it's pretty old it will not necessarily have a solenoid to operate the louvres. In the big old ones from 20 plus years ago - much better quality and not so much to go wrong - the louvres are actuated by switching on, where the switch operating mechanism has a flat piece of metal on top and when the cord is pulled, the flat comes down and lets the spring loading open the louvres. These do need cleansing, as in previous answers, but it may just be that the cover has been put back wrongly and it may be fouling the operating mechanism. If the kouvres break you might be in some trouble so treat them gently. They don't sell replacements for the old ones any more BUT; you can 'splint' the break with some slim spills of wood and two-part silicone or resin glue.

I've been there, done that and still have a fully operational (and clean) fan

docusk