Bad Blad

I was messing around with the big boy planning a photo project-well daydreaming about a project-well it suddenly seized up in a rather odd way. When I removed the film back it worked fine but as soon as I put it back on it seized up again.

Anyway tonight I gave it a good clean and surprise, surprise it's working again.

Could a single hair jam up the whole system?

Any advice gratefully received.