I suspect you're just loosening the valve core, instead of removing it all the way.
I have one of those, it really blasts out the air. You just have to make sure that you've unscrewed the valve core all the way before you pull the slide back and start dumping air. You can't see anything, so you have to do it by feel.
Screw the middle piece onto the stem. Then use the knob to (blindly) feel for engagement on the valve core. Once you feel the slot slide onto the core, unscrew the valve core until it is completely out of the stem. As the core starts to come out, it will start releasing air slowly. Once the core is all the way out, it really blasts air out.
Then after you've used it a few times, you'll sort of learn by feel/sound when the pressure is getting low enough for wheeling, slide the valve back and forth to check pressure/dump more air. When you're at the right pressure, leave the slide in and use the knob to reinstall the valve stem. This is another potentially fiddly moment done blind and by feel, but I've never had any issues. You just have to gently push the valve stem in using the knob, get the threads to engage, then screw it all the way in. Then you take it off.
It really does take a minute or so start to finish to dump air from 40 psi down to 15 once the valve core is all the way out.