The key card is present but its support is thin.
Prioritize useful support such as Endrider Catalyzer or Goblin Surveyor.
Gastal Thrillseeker starts speed before its entry damage resolves. On later turns, activate Apocalypse Runner before blockers to send a small creature through, then increase its power after the ability resolves if useful.
Your deck enables the plan but is losing the early turns.
Take a castable early creature or efficient answer before another expensive payoff.
Apocalypse Runner needs power two or less both when targeted and on resolution. A later pump is fine; activating only after blockers are declared is too late to evade an existing block.
The support is missing
Favor independently useful creatures and interaction if the narrow support does not arrive.