Mine stopped working also, it probably has a bad seal. Even the smallest gap in that seal, and they will stop working. Take a look at it, if it doesn't have any major cuts or breaks in it, try some vaseline. I'm serious, it worked for me. Rub some around the edges, this will help make a better seal with the top of the air cleaner.
It's vacuum activated, so the seal is the most important thing, if that doesn't work, just try opening and closing the hood several times, it's very tricky. I finally gave up on mine all together, if it is bad, good luck finding another one. I'm about ready to sell mine, it's been playing 2nd fiddle to my 69' SS/396, and it's just taking up space around here......KZ