Hi atavus,
You don't say where you are (I know: 'on the internet'.) The Typewriter Man, here in the UK, supplies good ribbons. And Rymans sells plain black Pelikan ribbons that are really good. If you buy the cheap ones off the internet you will get cheap ones - it's like buying generic refill ink cartridges for your printer.
But sometimes it's just the typewriter. I think there is a sense in which some machines get old and tired, and something maybe slips, the slugs don't hit the platen as hard, or there's some minute adjustment that loosens...? And type faces get worn. Especially if their user all those years didn't use a backing sheet.
But I've often found that if you clean the type it sharpens up the impression. Cleaning the segment makes a big difference. Cleaning it really thoroughly, under and behind and inside. Given the state we are getting these typewriters in, the single biggest problem they have is neglect. So, dirt, rust, dust, old caked ink, old caked oil. The number one panacea I've found for almost everything is cleaning and cleaning and more cleaning. (And don't buy cheap ribbons.)