It's a proximity thing. If you're far enough away from the pot, it won't catch you. I found this out on Corneria, when I jumped to the top while my friend ran way as far as possible. I was near the large hump , basically a little to the right of the middle of the stage, and I couldn't get him in it.
I've since tested it on multiple stages against multiple characters, and the results are the same. There's a certain radius you have with the pot, and anything outside remains uncooked.
To give you a better idea of the radius, I just jumped on the game and went to Pokemon Stadium (The first one, not 2) and put Kirby in the middle of the stage, right in button of the Pokeball on the floor. Then, I put player 3 (Wolf) over on the near the edge of the right side. On that white panel. Only Final Smashes on, no other items or players. If Kirby stays on the ground, jumps once, or jumps twice, and then used the Final Smash, he was still within range to grab Wolf. When I did 3 jumps, I couldn't get him in the pot. I've tested it with Kirby facing both ways, and even with Wolf facing both ways, but nothing I saw pointed to anything other then Wolf's distance from Kirby.
Hopefully that helps out :D If anyone wants me to test other theories out, let me know, but I'm convinced it's the distance between Kirby and his would be meals.
Edit: To answer Dodger, I think you just had some low-percentage opponents ^^ We haven't played with items on, the only item on when we do of course being the Smash Ball itself, and I never have had the situation you described happen to me.