I know what you mean. Sometimes, the same happens to me. It's some sort of bug but I was never able to exactly determine the steps to reproduce it (and so also never reported it). Most of the times (99%) works fine for me, though. But if you can reproduce it, then you should report it to e-on (on their site -> customer care -> contact a technician -> put your details and click "contact a technician" -> put more details and submit the bug; you'll be contacted some time later, normally within 24 or 48 hours, working days only).