I don't know the details since electrical is not my thing, but our electrician had this problem with new junction boxes for two of our pendants that need to be flush with the ceiling and had flat canopies. He said there is a different type of box to use for those that sits slightly recessed from the plaster (and he still has to come out and fix this, ugh!) but I don't know what it's called. The standard boxes, which worked fine for all of our other fixtures because they were designed to sit over a junction box, aren't quite flush with the ceiling.

Will this work? Not sure where the junction box is, above the ceiling, or below the surface. This has a shallow lip and may fit.

Here is a link that might be useful: Plaster box

We used what I think are called pancake junction boxes, because some of our pendants needed to be mounted on a floor joist. I know very little about electrical, but you may want to look into it.

The shallower boxes are called pancake boxes and you can get them to support regular light fixtures and heavier fixtures alike (ceiling fan rated). I've used them in my old house a few times- they are very handy, but don't have much space, ie can't put too many wires in there.

I have had to shim a couple of them at my house, to fit the uneven rafters/ceilings.....tricky, but can be done if your wiring is NM. I would not recommend that with the old k&t wiring- but that's a question for an electrician. I am currently re-wiring my old house top to bottom, and these boxes have allowed me to be a bit 'creative' with installing fixtures in areas they previously weren't, without tearing out the plaster, etc.

Here is a link that might be useful: ceiling fan rated pancake box

Thanks, the pancake box is just what I am looking for, and fortunately our wiring is NM, although maybe it would have been easier to start with k&t and re-do it ourselves...