It continues to amaze me that such problems are not more widespread.
If you are bored, desperate or just feeling in need of cheering up then stand and watch people use the barriers and Oyster.
At least 25% can't use the barrier; not holding the card on the reader long enough, trying to get it read through layers of other things, in proximity to other cards and best of all, obstructing the sensors.

Many blame the system. Mostly it's not knowing how to use a barrier.