Dawn of War Codex

Sometimes when I am using an ability such as execute or apo heal, I will try to click the ability on the target unit card. Often, I will get the error sound and won't be able to cast the ability, even though the squad is a valid target - I have gone immediately from not being able to target a unit through their card to successfully casting it by clicking on a model from the squad.

Does anyone know what causes this?

I run into this same problem too pretty often. No idea what causes it