I just can't remember tunes without words, until I'm playing them. What I mean is, I know any number of tunes where I can back up a fiddler or mandolin player, and I'll know every note - but I can't remember them on my own.
Ever been in a session where everyone is like that about a particular tune you're trying to play? You all know it, and you know you know it, but noone can remember how it goes - and then suddenly someone gets a bit of it, and you're all away, playing it together. Can be fun, but nervous fun if there are people watching and listening. (though if there's not too much hesitation, it can be quite impressive...)