There seems to have been some problem at Shepherds Bush Overground this evening which resulted in the barriers failing to open when using Oyster PAYG. Went to touch in and got the 'seek assistance' message. Tried again with the same result. Knew I had plenty of credit. Spoke to the station attendant who asked what kind of Oyster I had,I told him pay as you go and he opened the barrier and said to go through. I argued that I would probably get a maximum journey charge when touching out at my destination and he said if so phone up the helpline. Whilst this was going on a woman came along with a travelcard on her Oyster and the gate opened normally. Got my train and touched out expecting to see the maximum charge but it deducted the correct fare and everything appears to be in order so I don't know what the problem was.