Rpg Maker Developers Group

So, I'm trying to make an attack that does damage and heals the user (for less then the damage is output at, so drain HP as a damage type won't work, to my knowledge). But despite all my fiddling I can't seem to get it to actually do damage, it just misses (or does no damage if set on certain hit). I've ensured the enemy I'm testing on doesn't have high def (it has 1, the player has about 20 atk IIRC) The formula is as follows...

change_hp(a.atk - b.def*2); a.atk - b.def

As far as I'm aware, everything is in order. Damage dealing part of the formula is on the end of it, change_hp accepts formulas to my knowledge (and changing it to a flat number doesn't help, either), damage type is physical, it's set on HP damage, all that seems to be in order. Completely lost on why this isn't working. I've included a screenshot of the skill entry in RPG maker vx ace, to ensure I'm not missing something obvious, because it feels an awful lot like I am at this point. Thanks in advance! Protected download