The key card is present but its support is thin.
Prioritize useful support such as Clockwork Percussionist or Unsettling Twins.
Build small bodies before attacking with Arabella, then count creatures with power two or less as her trigger resolves. Midnight Mayhem supplies three such bodies and a lifelink attack when cast before combat.
Your deck enables the plan but is losing the early turns.
Take a castable early creature or efficient answer before another expensive payoff.
Increasing a creature above two power before Arabella's trigger resolves lowers her count. Defaced Gallery's pump is useful, but stack simultaneous attack triggers to resolve Arabella before the power increase when needed.
The support is missing
Favor independently useful creatures and interaction if the narrow support does not arrive.