Then you're doing it wrong. Maybe you've got a shitty belt, or you're not wearing it tight enough, or you're not performing the valsalva maneuver correctly, I don't know. But a belt helps. And if you're not performing the valsalva correctly, that's the first thing to work on.
It doesn't matter whether or not you "feel" that your abs are the limiting factor. Hell, even if they're not the limiting factor, stronger abs will always help your lifts, especially the press. This is why a belt helps.I worked up to doing planks with a 20kg plate on my back. and a good few knees2elbows. I dont think I could do any of this stuff anymore! But I dont think abs are letting me down in the press. It is my chronically small shoulder muscles.
And planks are shitty.