that is a common problem with that series of machines. They were "self lubricating" and after about 30 years, the lube gets very hard and starts to gum up everything. Serious cleaning involved to get it all out, and then they need to be regularly oiled like any older machines.
Do not try to force anything - they are plastic and it's very easy to break something when they get gummed up. they are great machines, but one of the more difficult to deal with getting to work again because of this.