Not sure how popular this attachment is, but it is quite loose with a full inch of vertical movement. Am I doing something wrong here, or should I just change this attachment altogether?
platic molle by Vito Corleone, on Flickr" />

That’s a common problem with narrow MOLLE belts and pouches that use rigid mounting systems. MOLLE is design for the attachment straps to be interwoven between the pouch and belt / vest which is what provides the secure, stable mount. When you can’t weave the straps back into PALS on the pouch, it allows them to slide around. On a narrow belt like that, you need the strap to weave back into a horizontal PALS row on the back of the pouch that will fit between the two straps on the belt and prevent the pouch from sliding up and down. Even if you had a plastic mounting clip that was shorter (like one that used three MOLLE rows), the narrow PALS on the belt would allow it to slide up and down.

If you look at the image showing the back of this ESSTAC KYWI pistol mag pouch, you can hopefully envision a flexible mounting strap, like a MALICE clip, going through the top loop on the belt, then weaving back into the middle row on the back of the pouch, and then through the bottom loop on the belt. That would prevent the pouch from sliding up and down on the belt. Those rigid plastic clips are quick and convenient but don’t work in this type of application.

Very good explanation.

If that's a G-Code clip/pouch, they make three different attachment clips. One is designed for shorter/not as wide MOLLE belts.