From what I have observed most chains just ride over the roller whatever you try to do to make it rotate freely, even if it spins freely at no load it will probably not do so under pressure.
I had a sticking bow roller once on a previous boat. Always had trouble retrieving anchor - would jam at the worst possible times, and I'd be floating about trying to sort it out... loads of stress. So in my experience at least you are right to try and sort it.
Unfortunately no idea how it was freed up, but never had a problem once it had been.
If it's tufnol, then oil will make it expand on the pin; if it's nylon, grease will make the sheave brittle over time; otherwise use a lithium / MSO2 pressure and water resistant grease to cope with the exposed environment.