I'm not that tech savvy but isn't the technology the same as when make you make a contactless payment? If so, why do so many have issues getting into the ground? seems to be random people at random times as well.
It's a bit of a UX disaster to be honest.
The big slot looks like where you *should* put your phone, so some people will try to do that.
Then the implementation means that the system will need to check you have a valid ticket, so it takes time to go to the server and check. Meanwhile there's absolutely no feedback that anything is working, so people will give up and move their phone away.
These aren't issues if you go every week and are used to it, but if 5% of 55k get it wrong, then it slows it right down.
Plus the occasional actual issue.