Thanks,Rich, and Slingshotx.
The frustrating thing is that the lane that does work works perfectly, and the PC-Doctor program says the parallel port is working normally, so I suspect this is some kind of Driver issue, but I don't know enough to start fiddling in this area.
I will try the serial port when I can get a lead and a socket.
A member of another American site, a while ago, suggested that this kind of problem is 99% of the time caused by pins locking out because they interpret a contact signal as a "short", and that the fix is to put a 150 ohm resistor in each detector circuit.
Unfortunately, he didn't say how to unlock the lockout - and I appear to be in the 1% where it isn't the fix,as I've tried it and it hasn't made any difference.
I do take the point about usually needing some protective circuitry, but as Brent Carlson is so emphatic about which pins not to short out,I thought I might get away with it.
The thought did occur to me that LT2000 is quite an old program, and might be better-suited to an older version of Windows than XP Professional, which is what my old laptop has on it.
If I succeed in boiling the serial port, I really will throw in the towel and buy Trackmate and a new(old) laptop.