Removing knockouts in old work metal boxes

Yes, knockouts are often one direction only, esp. in outlet boxes. If you can see that the KO is not even with the rest of the box, you need to take a something like a flat blade screwdriver, put it in the recessed side and give it a couple of medium hits with a hammer. Then once you get it started, use a pair of pliers (whatever type is best, maybe needle nose) to pry on the KO until it breaks loose.

A lot of handy boxes and old work "gem" boxes have knockouts that are made to knock from the outside in. This picture of a Steel City handy box shows it clearly

If you have to remove these from the inside, there is a trick. You can't pull the KO with a hammer. But if you drive a small self drilling screw into the KO from the inside

then back it out a few turns, you can pull on that screw with pliers to remove the knockout. If you can't get it with pliers, try a mini prybar

Yes, they are one way unless you really want to bend up the box. When the box is installed, I usually take a small screwdriver, slotted, and tap it into the crack between the knockout and the box and twist the knockout out a bit. Then grab it with a pair of needle nosed pliers and twist it completely out... be careful not to damage the insulation on existing cables when twisting out the knockout. If there is room between the wall and the box, you might be able to get a small screw driver in there and tap the knockout into the box and then grab it with your needle nosed pliers.

I recently had this issue and solved it this way:

Drill small hole in the center of the desired knockout (from the inside).

Drive a long self-tapping metal screw into the newly drilled hole until it is firmly in place.

Take a claw-hammer or kitty-paw to pull on the screw until you can get pliers onto the knock-out enough to twist//work it out.

I've gotten a few out with just a flat head screw driver at an angle and using a hammer to tap it into the crack or to make a gap. Once it's in, you can pry or twist with the screw driver to widen the gap and get something bigger in to finish the job.

It's not exactly easy, but it works without extra hardware or a drill.

And yes, like the others say, they are intended to be one-way from the outside in. I'd have to assume they didn't want the knockouts to be accidentally knocked out while the installer was putting wires in or a repair being done. Trying to fish one back into place would be even harder than trying to pull one out.