Its normal to see a real active fermenation at the beginning, but after awhile all the easy sugars are gone so the yeast slows down on the munching to spend extra time with the more complex sugars.
Also the airlock is not an good indicator of active fermentation even if you don't see any movement in it, since you can easily have a leak around the lid of your fermentor that is lettign the CO2 out. Best thing to do is to wait about 14 days take a hydrometer reading, note down the FG, wait 2 days, take another hydrometer reading (pulling a seperate sample of course), and if it is stable then fermentation is done. If your first reading is near your OG and you don't see any sign in your fermentor of fermentation (no kaursen ring) then you might have an issue, but we won't get into that now.