Why doesn't this monster's onhit spell work?

Ok, so I've got this custom monster, the "Diggle Vampire Spawn" which is supposed to cast a spell when it attacks that heals the monster. For some reason though, it doesn't ever seem to cast the spell when it hits, even when I increased the casting chance to onechancein="1"

Since I can't figure out what's going wrong with this, maybe someone here can. Here are the stats for the monster and the relevant spells:

<monster name="Diggle Vampire Spawn" level="1" taxa="Undead" tiny="1" diggleHell="1">

<idleSprite left="sprites/monster/vladiggle/vladiggle_run_l.xml" right="sprites/monster/vladiggle/vladiggle_run_r.xml" up="sprites/monster/vladiggle/vladiggle_run_u.xml" down="sprites/monster/vladiggle/vladiggle_run_d.xml"/>
<attackSprite left="sprites/monster/vladiggle/vladiggle_atk_l.xml" right="sprites/monster/vladiggle/vladiggle_atk_r.xml" up="sprites/monster/vladiggle/vladiggle_atk_u.xml" down="sprites/monster/vladiggle/vladiggle_atk_d.xml"/>
<hitSprite left="sprites/monster/vladiggle/vladiggle_hit_l.xml" right="sprites/monster/vladiggle/vladiggle_hit_r.xml" up="sprites/monster/vladiggle/vladiggle_hit_u.xml" down="sprites/monster/vladiggle/vladiggle_hit_d.xml"/>
<dieSprite name="sprites/monster/vladiggle/vladiggle_die_r.xml"/>
<castSpellSprite name="sprites/monster/vladiggle/vladiggle_wand_r.xml" />
<sfx attack="diggle_attack" hit="diggle_damage" die="diggle_die" spell="diggle_cast"/>

<digSprites downSprite = "sprites/monster/digula_digging/digula_dig.xml" upSprite = "sprites/monster/digula_digging/digula_pop.xml" /> <!-- These are custom sprites cobbled together from the core digging and digula sprites -->


<info latin="(Poingus Poingus)" text="A strange little bird-thing that tunnels through walls and feeds on the blood of the living."/>


<dig mindistance="3" percent="15" ambushpercent="15" blockedpercent="100" minturns="3" maxTurns="6" />

<stats numFig="2" numRog="1" numWiz="2" xpValue="20" />
<secondarybuff id="4" amount="-3"/> <!-- crit -->
<secondarybuff id="6" amount="4"/> <!-- dodge -->

<palette tint="140"/>

<resistances necromantic="2" toxic="1" asphyxiative="1" righteous="-1" />
<damage slashing="1" piercing="2" necromantic="1" />
<onhit spell="Weaker Vampiritic Attack" onechancein="1"/>

<ai aggressiveness="4" span="10" spellPercentage="5" />
<sight cone="90" modifier="1.10" />
<spell name="Lingering Weakness Missile"/>
<spell name="Lingering Dullness Missile"/>
<spell name="Weak Existential Despair Missile"/>


</monster>



<spell name="Vampiritic Healing Effect" type="self" >
<effect type="heal" amount="2" amountF="0.05" />
</spell>

<spell name="Weaker Vampiritic Attack" type="target" >
<effect type="damage" necromantic="1" necromanticF="0.06" primaryScale="3"/> <!-- Scales to caddishness -->
<anim sprite="sprites/sfx/bloodmagic/bloodmagic" frames="7" framerate="60" centerEffect="0" sfx="fleshbore" />
<effect type="trigger" spell="Vampiritic Healing Effect"/>
</spell>