I brewed my 5th homebrew this Saturday, an all-grain American stout. By Sunday I had very minimal bubbling. Monday there was a bit more but far less than my previous batches. Today there are none. I've never had a batch bubble this little. Typically in 24-48 hours the bubbling is pretty intense. Not so with this batch. I rehydrated dry yeast (Nottingham) and pitched at the manufacturers recommended temp. Should I re-pitch more yeast? Is my brew bad?
Bubbles are an inaccurate way to gauge fermentation. Can you see if the beer is fermenting? Have you taken a gravity reading? It's also possible that the temp was on the high side, fermentation finished quickly and you missed it. Look for physical signs. And in homebrewing you can almost never go wrong by just waiting longer.
The other answers here are spot on. I have one thing to add. Danstar Nottingham can be a freaky fast fermenter. A two day ferment is not out of the question for this strain. It might just be done.
I always have Nottingham on hand in case some other strain gets stuck. That stuff is one beast of a yeast.
There could be a leak somewhere that allows the CO2 to escape.
I have a pale and a speciality that is fermenting next to each other; the speciality sounds like a machine gun and the pale has barely made a bubble. I had the same thoughts as you, but I simply opened the fermenter and saw a nice krausen on the beer, so I am happy, but now I know that I have a fermenter that I have to check for leaks.
Leave the beer for at least a week and then take a gravity reading. If it has not dropped at least 50% then you have a problem.