I am not 100% but you may need <Cost>1</Cost>  in your trait.  I looked thru a couple in the game's /data/english/coreabilities.xml I was looking for something <Sovereigns> or something..  but armorer/beastlord/hunter/etc.. all have that Cost tag. 

Also make sure i'ts in the right category within the xml. Just adding a cost to it won't do the trick. This is an example of what I did to add a trait to the options

<AbilityBonus InternalName="Champion_Weakness">  This is what your ability should appear under if it's a weakness. There is another one for positive traits
<AbilityBonusType>Champion_Talent</AbilityBonusType>
<AbilityBonusOption InternalName="Frail">
<DisplayName>Frail</DisplayName>
<Description>-1 Hit Point per Level</Description>
<Icon>Ability_Frail_Icon.png</Icon>
<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_HitPoints</StrVal>
<Value>-1</Value>
<PerLevel>1</PerLevel>
<Provides>-1 Hit Point per Level</Provides>
</GameModifier>
<Cost>-1</Cost> You will have to add this
<HideWhenUpgraded>0</HideWhenUpgraded> and add this
<Type>Weakness</Type>
<AIData AIPersonality="AI_General">
<AIPriority>5</AIPriority>
</AIData>
</AbilityBonusOption>