Make sure you have depressed all the pressure plates, and Oswald has zapped them all. (I use anvils). After you have completed all three siphons, go back to the original siphon, that had one pressure plate. There is a projector screen at the top of that siphon. (Do not take the one you arrived through, that is on a ledge.
There are two ways to get there -- one is back through the original tunnels, and the other is through a cave that was opened.
Get up to the top of the first siphon -- either by climbing the water spouts or climbing the pistons.
I think maybe you misunderstood what I was trying say. Sorry about that.. Let me try and explain a little better....
Ok, there are 3 different guardian siphons that need to be disabled. There are two ways to do this - either plug up the air vents with parts acquired by thinning cars (which destroys the siphon) or depressing the pressure plates in the ground and have Oswald zap the consoles (which reprograms the siphon).
What I did was use both ways...which is where I think I messed up.
On the first guardian siphon, I plugged up the air vents with parts acquired by thinning cars. This disabled the first siphon.
On the second guardian siphon, I also plugged up the air vents with parts acquired by thinning cars. This disabled the second siphon.
Now on the third guardian siphon, I depressed the pressure plates (using the anvil) and had Oswald zap the consoles. This disabled the third siphon.
After I made my way back out to the first area, Oswald tried to free the Guardians, but Gremlin Jamface said that even though we disabled the guardian siphons, the guardians seemed to be "stuck"... I still received a trophy for disabling the siphons.
Maybe because of my decision to use both methods, it caused me not to be able to free the guardians and there is nothing I can do about it.
- Plugging the vents will blow up the main Control Tower, keeping the Guardians trapped and creating a 'staircase' out of pipes to the World of Evil.
- Using the pressure plate method, the Guardians will be released, restoring everything, and Paint will gush out of the Guardian Pool, allowing you to reach the World of Evil, grabbing a Generic Pin on the way.
As there are three siphons, the Guardian's fate is a 'best of three' rule; that is, whichever one you use the more of, pressure plates or vents, will determine your reward. Even if you use two of one method then use one of the other, the outcome will still be that of if you continued to use the original method.
In your case, as you used the vents twice, no amount of pressure plate usage could override that fact, keeping the Guardians trapped. Here's to a second playthrough, then, eh?
My advice is to just get on with the game. What you did does not impede your ability to continue the game -- by getting to the projector screen at the top of the first siphon.
What you did has no effect on the game's ending, nor your trophies, challenges, or pins. It will change which activity you play, later, when you return to Autotopia, but you get/buy the same pins. No harm / No foul.
(Those are "guardians" that you did not rescue, not "trapped gremlins". )