I have experimented with this and found that if you are in combat while on your mount and dismount your pet is stowed and you can not call him out until combat is over. So not only does this happen when using the attack dismount (1 key) but it also happens if you get hit and dismount without attacking (IE the 'x' key). Soulbeast builds cannot use any of their unique skills (F1-F5) until out of combat. The latest patch has turned my pets into the incredible hulk post meeting Thanos. He just wont come out in a fight.