Having trouble with dungeon generation using level instances as rooms

I’m trying to set up a dungeon generator, that will generate a maze-like floor from a set of rooms. Said rooms are specific levels instanced into the main level.

Although the bulk of my Blueprint runs just fine – though not perfectly – the last bit is giving me nightmares at this point.

The thought process and basic algorithm is simple, really. Get a random free spot, spawn a boss in there. Grab a different free spot, spawn a treasore. Grab yet another and spawn a shop. Grab the rest of free spots and spawn generic ending rooms there.

The result I’m getting, however, is that only a treasure room and generic rooms spawn. Or only a shop. Or a boss and a shop. Or – rarely – all three, as it is supposed to be.

The whole issue, in more detail and with screenshots of my graphs is in this Imgur album.

I’ll be grateful for any input. It’s the only issue that’s holding me back at the moment, and it’s been holding me back for the longest time. Over a month now, I believe.

It’s been through many, many iterations, each one makes it work just a bit better, but this is the issue I am simply unable to solve. Nothing I’ve tried works.