"Malcolm & Nika" wrote in message ...
Its the magnetic strip getting wiped by something. Common cause is a
magnetic clip on a wallet of handbag where it is kept. Another common one is
the magnetic bracelets the older folk tend to wear for joint relief etc. Can
also be done by mobile phone in close proximity. Happens very regular.
Cant it now be done at your local station. It was introduced like that about
2 years ago but it may have changed perhaps. Just ask the nice young lady on
the barrier to see if it can be done locally.

Unfortunately for the OP, he said 'plastic', which means that he has
an Oystercard.

In the case of an Oyster, exposure to powerful radio frequency
emissions or other EM radiation could damage the chip and/or aerial
within the Oyster. Stuff like microwaves, TVs, cellphones or carbon
arc lamps (!) could be responsible.

One likely culprit are the RFID-busters used by department stores,
which send out a superpowered radio burst to fry the chip inside the
disposable RFID tag. I wonder if the OP works in or near a department
store.