Problems feeding vu-foil material with LJIIP

Paper feed works fine, both from the LC tray and the MP bit. However, I frequently need to print
vu-foils (OHP acetates), but I'm now having feed problems. The acetate material is exactly the same brand
as I've used for the past six or more years, and I doubt that it's been reformulated.

I *used* to be able to put a supply of sheets of acetate, each interleaved with a sheet of ordinary paper, into the LC tray
and then print two copies of each page. That tidily gave me a legible hard-copy backing sheet of each OHP slide, and also
avoided the problem of the occasional pick-up of two (or even more) sheets of acetate, which gets nasty!

Now I find that, although the acetate prints perfectly, the printer /thinks/ that it hasn't, producing the ubiquitous
Error 41.

So I switched to putting the acetate stock in the MP Tray; again, in the past I've managed readily to put a stack of sheets
in there, and the printer has picked them up and processed them one-at-a-time. Now I find that if there is still a supply
in the hopper, I get the same Error 41 immediately after printing each sheet. (And of course, Alt+Continue reprints the same
page).

My only solution is to place the sheets on the hopper one at a time: it's fine like that. But I *must* wait until each printed
sheet has been fully ejected, and the flashing On-Line light extinguishes, before placing another sheet in the tray: otherwise
I get Error 41 again straightaway.

I'm sort of theorizing that there's some sort of failure properly to detect the passage of the trailing edge of the sheet.

But why?