I thought mine was done too...racked to secondary carboy and got another krausen ring a day later...
A full week in secondary, krausen is STILL on top and it's bubbling every 6 seconds or so.
Maybe your yeast got bored. Go to secondary after a few more days and see if the mixing and sloshing around from bucket to bucket don't restart your ferment.
Also depends on how big a beer it is. If it was a 1.030 beer, it doesn't have to do much work to be done. Mine was a 1.070, so I knew it would be a bit longer fermentation process to get it down into drinkable range.