Don't worry about it, and Don't use your airlock as a fermentation gauge...
Over 50% of my beers never have a bubble, it is normal actually for some not to bubble...
That's why we say you should never use a cheap chinese plastic airlock as a "fermentation Gauge," it's not...It's an airlock, nothing more, a VALVE to release excess CO2, to keep from blowing the lid off the fermentor...If it's not bubbling that just means that there's not enough CO2 to climb out of the airlock, or the CO2 is just forming a nice cushion on top of the beer like it's supposed to, or the airlock is askew, or it is leaking out the cheap rubber grommet, or you have a leak in the bucket seal...all those are fine...if CO2 is getting out then nothing's getting in....
That's why we recommend to new brewers to use their hydrometers, not their eyes. I believe MOST of the new brewer's "stuck fermentation threads are not, stuck, they are using the airlock as a "gauge" rather than checking gravity.
I have found that oldschool S types seem more reliable than the 3 piece to actually bubble. I think maybe co2 isn't always heavy enough to push up that plastic piece in the center of the 3 piece.
Also...Since you are dealing with yeasts, which are living organisms there is a huge wild card factor..meaning no two fermentations are ever the same..so comparing or trying to compare one to another is really useless...think of yeasties as wives or children and you will realize that it is pointless to pin them down...that it's best to just trust everything is all right..
It usually is...