I used my card for the first time on the tube this weekend and almost without fail had to tap several times or use a second turnstile to get through. Error 99 Try Again came up on the screens.
Remember that when dealing with contactless cards on TfL gates, it’s more of a hold squarely against the reader until the green light shows rather than a tap. This can take up to one or two whole seconds on some types of readers under certain conditions. Any attempt to pull the card away too quickly will result in an error such as 94 or 99.
It’s worth bearing this (also from RichardR) in mind too -
Just to update, I’ve been informed today that a gate code of 99 roughly means “customer moved too far forwards in to the gate before presenting proper payment”. If you get this code, you should take a step back before retrying. It’s apparently a very common issue with the wider gates and people with pushchairs or large bags.
Edit: Some sources claim it can mean “Transaction timed out” or “Gate fault (timeout)” depending on the type of gate.