Usually I edit for flow first (rearrange, add/subtract), then focus on how to better articulate the piece. I'll usually do that by expanding on ideas or metaphors (I use them a lot). Lastly I go over it a few more times to make sure it's concise.
I honestly not know how else you could change your approach, other than giving yourself time away from the piece. I don't write a lot of poetry myself, but the processing of revising never changes much for me. I don't see why it would need to.
I guess if you're having trouble with a piece, you can stick it in a drawer, or do what I have done on occasion--make a cut-up out of it. Or you could write it in another style, etc.
It's not necessarily what hasn't been working, I'm just looking for ideas. Usually I'm only editing one or two poems at a time and will mill on them for a few days.
Well one, I would suggest edit them for a short period then put them away for a few days and then go back. In that time you'll be able to come up with new ideas. Also you can ask other to take a look