M R, My first WSM was the 18.5 and I'm trading a pellet grill with my daughter to get it back. I use Kingsford Blue Bag also. Your question will get a lot more answers if you post it on "New WSM Owners". What temp were you shooting at? How open were your bottom vents when the temp fell? Was your temp taken with an electronic thermometer or were you using the lid thermometer? I think folks will need a tad more info to give their best advice.
However, a few general guidelines. Assuming your bottom vents were partially open and top vent fully open a temp drop of 20°+ is a good indicator that it's time to open your lower vents more but not necessarily full open. When you do open them to 100% you need to monitor the rising temp and reset them at least 20° below your target. BTW, most folks have a problem with high temps on their first cook. If you look at Chris's cooking log on the TVWB site you can see what percentage he had his vents open. That's just a guideline and not necessarily what you'll need on your cook. Also, as you probably noticed it's quicker raising temps than it is lowering them.
Keep asking questions.