This has happend to me, I pitched the yeast, next day nothing, take the lid off and give it some O2 and away she goes. Wine has done that to me also.
But to be honest I think your lid was not all the way down, since your gravity was 1.026 It had been fermenting and and I dont think it just stuck with the OG at 1.047. I think somehow, some way the lid was not sealed good, or pressure was getting out someplace else. between the airlock and the stopper possibly. I would almost bet on it. You mentioned your activity was slow, it may have been fast, just loosing co2 pressure someplace else, then when the fermentation slowed it took the path of least resistance.
If you have a valve on your bucket once your done, empty the bucket put an airlock on it and blow slightly or put your finger over the airlock hole and blow on it see if there is any leaks.