OK, looking over the code real quick, I see two possibilities...
One, the Can Stack tag is set to 0, although I think that is to prevent you from casting it on the same stack 5 times...
Secondly, this tag seems out of place...
I'm guessing that this is the tag that is 'locking out' the spell. Note that it unlocks the same spell that you are casting (refers to the same spell internal name tag) Have you tried commenting it out yet?
Finally, if the above doesn't work, I'd suggest creating an entirely new spell, with just this modifier tag
Use another strategic spell that you KNOW can be cast on multiple units/stacks at once as your template, and simply delete the 'unrelated' gamemodifiers from that spell and add this one.
DO keep in mind that if you remove the 'cast once only' limitation, there is nothing to stop you from casting this spell on say five different Champions in the same stack, hence possibly adding a total of 5 to your army move... In my experience, having a playtesting Sov which has the AdjustArmyStat (+1 Move) tag, the Tireless March spell will 'stack' on top of that modifier.
What might be more interesting is to create a 'Mass Tireless March' spell, which has mod type Player instead of Unit. If I'm guessing correctly, that would add +1 move to ALL of a player's armies. Hence, it should cost a bit more to cast...
It would also stack with Tireless March, so one of those stacks would have a +2 move if both spells are in play...
Edit: Aha, this tag is new to me...
Interestingly, this is the only Spell that doesn't have
accompanying the <UniqueSpellUnlock> tag. So all other 'unique' spells are Sov cast only...
This is the first time I've noticed this tag (doing a Core Spells check, only a couple of other spells have it). Try commenting it out, plus commenting out the 'unlock' tag above ( <!-- --> ) and see what happens...