If they start hanging just a tiny bit crooked, they can get really hard to slide.
I think that's exactly what happened (I have similar issue with the deck door besides it's much lighter, so easier to deal with). I wonder why the BO didn't try to fix them, but oh, well...
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."