Thanks for the replies everyone
It seems it might not be the scribble that causes the rejection. The other day, I used my ticket through four barriers - getting into the Underground at Paddington, leaving the Underground at Liverpool St, getting to the platform at Liverpool Street and leaving Harlow Town station. It was a Carmarthen to Harlow Town ticket, and the ticket was successful at all barriers
apart from the barrier at Liverpool Street, where I was trying to get onto the platform to get on my train to Harlow. I showed the guard and he looked puzzled because he said he didn't know where Carmarthen was, and hadn't heard of it
Some days it's successful in every barrier, other days it's rejected at every single one. I try and keep my tickets away from my mobile phone, so I don't know what's causing it really.
I think the error code is 09, but I can't confirm for absolute certainty.