Step Functions

How Do i make the priorities only fire if the ability fires?
currently im testing a 1, 12, 123, 1234 step function priority but it does not fire by that order

/cast Execute
/castsequence Colossus Smash, Overpower, Mortal Strike
/castsequence Overpower, Slam
/castsequence slam

It should fire if available but it find key pressing to quickly skips the abilities
But it should go as follows

Execute
Execute, Collossus Smash
Execute, Overpower, Overpower
Execute, Mortal Strike, Slam, Slam