How come unit order triggers don't work with level 6 spells?

In all my boss battles I make sure the boss always has the specific spells and abilities I want them to have and then I said what level there abilities are...

Through out the battles I have many triggers like "Order the boss to cast blizzard on position of unit, or carrion swarm or whatever"
And it always works, never have any problem with it... But it never works with level 6 spells, ever. Not even if that unit knows that ability by default... My bosses are usually always level 10 and always have the one point spent in the level 6 skill...
I have tried it numerous times with numerous abilities. starfall, earthquake, death and decay... it never works...
On my one on one against Cenarius. I gave him starfall and order him to cast it at a certain time, but he doesn't... I even when as far as to make a night elven priestess hero unhide at that moment and cast starfall because that is her default ultimate ability and then have her hide again... But nope, that didn't work either...

Same thing when i tried giving Earthquake or death and decay to antonidas...