So Im doing reputations and currently farming the crap outta Thrallmar faction. When you do all three heroics and then change difficulty to normal so you can farm Shattered Halls the server will be unable to find the instance. Im not sure whats up with this, but it happens all the time. I don't know a fix either, maybe a soft reset?

Alternately I guess I could try farming normal until Im bored and then switch it to heroic for the finale three runs.