Airlock bubbling ALWAYS slows down...I don't know why new brewers always worry about it. But don't do anything. The beer's fine...
Airlock activity is irrevelent. Just gravity points on a hydrometer.
Airlock bubbling (or lack) and fermentation are not the same thing
. Neither is the rate of bubbling corresponding to rate of fermentation.
You have to separate that from your mindset. Airlock bubbling can be a sign
of fermentation, but not a good one, because the airlock will often blip or not blip for various other reasons
...so it is a tenuous connection at best.
Fermentation is not always "dynamic," just because you don't SEE anything happening, doesn't mean that any-thing's wrong,, and also doesn't mean that the yeast are still not working diligently away, doing what they've been doing for over 4,000 years.
That's why you need to take a gravity reading to know how your fermentation is going, NOT go by airlocks, or size of krausen, or a calendar, the horoscope or the phases of the moon (those things in my mind are equally accurate).
The most important tool you can use is a hydrometer. It's the only way you will truly know when your beer is ready...airlock bubbles and other things are faulty.
The only way to truly know what is going on in your fermenter is with your hydrometer.
Like I said here in my blog, which I encourage you to read, Think evaluation before action
you sure as HELL wouldn't want a doctor to start cutting on you unless he used the proper diagnostic instuments like x-rays first, right? You wouldn't want him to just take a look in your eyes briefly and say "I'm cutting into your chest first thing in the morning." You would want them to use the right diagnostic tools
before the slice and dice, right? You'd cry malpractice, I would hope, if they didn't say they were sending you for an MRI and other things before going in....
Don't think ever about doing anything like adding more yeast without taking a gravity reading.