Be sure roll call parameter is set correctly. I can't remember which parameter it is.
If I remember correctly there is a way to test the floor board comm with the terminating resistor on the last floor board. You take the terminating resistor from the last floor board and plug it in the first floor board in the string. Then do a LON download and see if shaft comm returns. If it does, you then take the terminating resistor and go to the second board in the string and repeat until you find your bad board.
Example for a 3 stop:

Machine Room -> [FCB1]=[FCB2]=[FCB3]=Ω
Machine Room -> [FCB1]=Ω (Leave FCB2 & 3 disconnected and do a LON download) if shaft comm returns check the next board:
Machine Room -> [FCB1]=[FCB2]=Ω (Leave FCB3 disconnected and do a LON download) see if shaft comm returns

Ω = Terminating Resistor
= Is wire connection
[FCB] = Floor comm board

Hope that makes sense. Also just curious, is it a V3F or KDL drive?
Is drive menu 6 accessible or does it vapor lock when you try to access it? (although you wouldn't think it could be a drive problem if it levels into the floor just fine)
I think you are barking up the right tree thinking a floor board could be bad.

Good luck