We have a tecalemit roller with a similar problem.

The n/s roller will only work if button is held, the o/s roller is OK. I figure it must be a problem with the wheel speed sensor. I keep meaning to get the scope out and check the signal, but never get round to it.
Occasionally both rollers stop working, this is always due to the wire to the hand controller breaking, usually within 6 inches of the remote. I just shorten the wire and re solder it to the switches. In fact done this so many times that last year had to replace the wire as it was getting a little short.

Tecalamit will not sell any spares to me directly, they will give me the part numbers, but I have to order parts from Andrew Pages.