When two incompetent crews try to rob her bank at the same time, an ambitious bank manager must play one against the other to buy time and save the money.

Great suggestion from DPG.

The only thing I can add is that the bank manager seems unrelated to the plot. In other words, if you were to replace her with any other manager the story would stay the same. This means that the plot isn't inherently an extension of her flaw and character. Perhaps it would be better to describe her as a genuinely flawed character and make the premise a result of this flaw.

What if she was the security guard or head of security at the bank, and it's her lack of professionalism or laziness that allows the robbers to break-in.

"When two crews rob the same bank, the beleaguered bank manager plays one crew against the other in order to save the customers."

Continuing dpg's line of thought, perhaps the manager should be more concerned with the hostages than the the money.

When two incompetent crews try to rob her bank at the same time, a quick-witted bank manager must play one against the other to buy time and protect the hostages.