A bit more info...
If I change the popup menu to show values from a single field, then I see that field. If I say I want to also show a value from a second field, it's then that it breaks.
See this screenshot.
If I untick "show value from second field" then the popup menu works ok (I get a list of ID numbers). If I tick the same box I get a single entry in the popup menu, number 1, and with a question mark (?) where I should see room names.
Check the storage options of the fields your are referring to as the source of values for your value list. Make sure that they are stored, indexed fields.
I think I've solved it. And it was me being a muppet. Here goes, and I hang my head in shame, because I must have done this several times...
1) I created a field called "roomName" expecting text, but left it as a numeric field by accident.
2) In regular data entry you are not warned of this, and you can happily enter text into your numeric field (hence the reason it didn't get spotted).
3) But... when you try to use the numeric field that contains text in a popup menu, you will get a single question mark.
Maybe if I was concentrating harder instead of watching Terminator on iPlayer while I'm working I would do a lot better!