All I can think of is that perhaps the people who've been uploading from you aren't sharing.... not really likely though, even if some aren't sharing you'd still expect a few to be. It's possible that all the people who've uploaded from you are offline at the times that you're searching but again... a bit unlikely?
Somebody else might know

I can find files uploaded from me... they appear with a ticked paper icon next to them, though of course I haven't downloaded them!!