When you activate one of the Jack-O-Lanterns, a spawn sequence of NPCs will happen on all players within a certain radius. If you have one of these sequences activated, but for some reason do not defeat all the NPCs that spawn on you, there will be a 4 minute delay before another Jack-O-Lantern sequence will spawn on you.

 

If you find that you aren't getting any NPCs when you're near a JOL when it is activated, then there is probably at least on NPC from a previous spawn left alive. Either defeat the remaining NPCs from the previous spawn (or have someone do it for you) or wait a few minutes. After the timer is finished, the previous NPCs will de-spawn and you'll be able to have a new one launched on you.

 

Have fun!