Zifa stampers were designed for "Aptis" tickets which were slightly papery, zifa stampers do bed in after a bit of use, the ink tends to be a bit stickier after about 200 stamps and stays on, As for stamping a ranger, it can be stamped once to show that the ticket has been used and does not invalidate it, if it hadn't been stamped then technicly the customer could say they hadn't used it and put in for a refund.
That makes sense. The current tickets are rather shiny. The printed ink rubs off them after a while so it is no suprise that the Zifa ink doesn't stick very well