I had some of the same issues with Oyster - apparently the problem is when you don't 'touch in' and 'touch out' with every journey.

When you touch in at Forest Hill, the Oyster doesn't yet know how much to charge you and therefore charges you the maximum. If you, say, touch out at London Bridge, then the Oyster is meant to automatically refund you the difference in fare. However, if you forget to touch out, you never get this refund and therefore the maximum fare will have been deducted from your card.

I phoned up TFL re this as I travelled on the Northern line from Angel to L Bridge, then from L Bridge back down to F Hill. I successfully 'touched in' and 'touched out' on the Northern line but the gates were open at London Bridge rail station so I walked through to platform 5 and didn't 'touch in' for the next leg of my journey. I was therefore charged the full max fare when I got off at Forest Hill, despite the fact that I'd already exceeded the Oyster 'capped fare' for the day. When I phoned TFL, they refunded me the difference, as they could clearly see that I was at London Bridge tube station 20 minutes before before I arrived at Forest Hill. But it's worth checking your PAYG balance from time to time, as the only reason I even noticed was that I needed to 'top up' my balance sooner than I had expected.

I also had problems getting a one-month travel pass added to my PAYG Oyster. As other people have said, FH station was unable to help and said that I needed to travel to a tube station to get it loaded onto my card. Since I otherwise didn't need to go to a tube station that week, I phoned up TFL again and found out that you can put 1-month travel passes onto your card via the Internet or phone. Hopefully this will save some of you an unnecessary journey!