Everytime someone changes his/her role while already grouped, the group finder (GF) gets disoriented and starts looking for the wrong roles. For example, even though you started it looking for 2T 2H 8DD, if someone joins as a tank then quickly swap back to DD (wich people do because pug DDs will be pug DDs), the GF starts thinking you're looking for 1T 2H 9DD.
Right now, the only way to fix it is to get out of the instance, wich works I guess