To amplify what my friend Esme said:
It's very important to understand what that NG tube, probably a Salem sump tube with the little blue pigtail, is, how it works, and why we care.
Before Salem sump tubes were invented, all we had were single-lumen tubes to suck out stomach contents. What's the problem there, you say? Well, if suction has removed all liquid stomach contents and it keeps sucking, what you get are little punch biopsies of the stomach mucosa, and this is not a good thing. So, some clever person (perhaps a Salem witch) figured out that if there were a way for the suction to have something else to suck on, to break the vacuum in the stomach, like air (and any other fluid that happened to accumulate too, of course), then this would protect the stomach from having vicious little hickies from suction at the eyelets down below and bleeding and such.
The air vent on the salem sump is that solution. Air should always be going down the blue lumen. How do you know if that's happening? Well, you can put your ear near it and hear it, or put your thumb over the end of it briefly and feel suction there, but the best way to see is that there should always, always, always be air bubbles coming up the main lumen to your suction cannister. Now you know something important: if it isn't, there's hickifying going on down there, and that's a bad thing.
The air vent lumen should always be patent (open) all the way down. If it isn't you can do a couple of things (after you have determined that some fool has not turned off your suction outlet). One is to untape the tube and pull it back an inch or so, and then put it back. You can also push some air (not fluid) down the blue lumen. Yes, you may have to do this all dang day long, but you will often be rewarded by a gush of gastric goo, which is what you wanted anyway.
What's going on if there is no air coming up the main lumen and there's goo coming out of the air vent? Well, (assuming your suction is still turned on and plugged in properly) your main lumen is probably occluded and goo has nowhere else to go but up the air vent, and you should probably think about replacing the whole tube. But before you do that, if it's not otherwise contraindicated, shoot some NS down both lumens, and follow that with air down the air vent lumen. See if that doesn't get things moving properly. If not, do the wiggling thing as above. If that doesn't work, get the ok to replace it (or have the physician or NP come and replace it).
Some Salem sumps come with little plugs in the blue lumen. They're supposed to allow air entry and prevent backflow but they don't always really do a great job of it. If you can still verify air entry past that plug and bubbles are going up the main lumen, it's fine. but if it's gotten grubby and acts like an actual plug, get rid of it, or what you have is now a gastric mucosa-grabbing single-lumen tube.