I'm not familiar with the Roland VP540 or the Summa T750, but my Mimaki sensor went out of alignment once & couldn't detect media width... so I bought a new one for $35 & when I went to put it in, I discovered mine was just aimed wrong, so I straightened it out, tightened it up & "poll size" started working again.
Other than that though... I'm not sure what it could be.