My classes have improved a lot in this respect. I also teach middle school and I can totally relate to how your classes are going. Having said that, the last three weeks have been much better. I think the reason is that I'm focusing on very simple dialogue/vocab/expressions. I lay out a simple dialogue on the board with several options for each part. Then I leave a little wiggle room for creativity. The ones can't/won't try to create their own stuff just use the formula and piece together dialogues. The ones who are much better create (sometimes completely) their own dialogues. They are starting to realize to that making their own is much more fun.
Keep at it. Try to stay positive. You'll go crazy if you don't.