Blocked tubes

So after a long and painful time trying to get an iud, my gyno suspected a UU after a vaginal ultrasound. Go in for an HSG, no UU but they had to use extra dye just to get my tubes to light up even a little, meaning my tubes are blocked, that was the "official diagnosis." But no one has told me, or can tell me why they are blocked or what's blocking them. Next summer I go back in for a yearly check up, and she said we'd talk about my fertility more if I'm not pregnant by then, because I'm not on BC, although we aren't trying trying. I'm just wondering what kinds of things can naturally block tubes, I'm not finding a lot of helpful stuff online.