To repeat what others have said: It's completely normal. Have you tried the same with store bought beer bottles? I know I have, and although they possibly keep the carbonation somewhat better, the same principle apply.
The trick with adding a little bit of sugar MIGHT work, but if the beer has matured enough there should be more or less no yeast left. As I'm sure you know, carbonation happens when the yeast in the brew "eat" the sugars, producing CO2. If there's no yeast left, there is no carbonation. (This is sometimes a challenge when brewing very strong ales, with lots of sugars - when it comes to bottling, one might find that there is too little yeast left for good carbonation, hence resulting in a flatter beer than you'd probably prefer. There are tricks for dealing with this, of course.)