How can I motivate my ally to leave home?

First rule of Warhammer: Do not bother with alliances unless you can confederate the ally eventually.

Alliances themselves are more a liability than useful. You can get dragged into wars you don't want, the ally may outlive their usefulness and you have a knife in your back, you can't attack for many turns or have to tank your reliability rating. Never ever go further than trade agreement and non-aggression pact with any AI you cannot confederate.

Allies themselves are also very unreliable. Whether your particular situation is a bug or Clan Mors simply has no interest in moving their armies at this time remains a mystery. Coordinating war targets is nothing more than a suggestion for the AI, whether they follow it is up to them.

Do they share a border with the faction you are at war at? If not, this makes it very unlikely they'll bother to engage them.

So, long story short, you cannot motivate your ally to do what you want, they'll do what they want.

One way to get your allies to utilize their armies is to give them a War Target through the diplomacy menu. The War Targets can be either an enemy settlement or an enemy Army. Once you've given your ally either command they should send out an army to attack the target.