User blog comment:Omega natsu2/Chapter 371: Tartarus Arc, Part 2: Song Of The Heavenly Dragon, Predictions./@comment-86.70.179.7-20140210084523/@comment-7418318-20140210142103

Because she never really fought against the Dragons. When she tried to get to the pedestal the Dragons seemed unaware of her, so summoning a spirit to attack them would have been a very stupid thing to do.

Then Zirconis caught her with his magic and grabbed her before she summoned any spirit. Only when she and Yukino tried to destroy the Eclipse Gate might the new key have come in handy, but she decided to go with Taurus and another Zodiac (could be Saggitarius from the look of the key), instead of experimenting with a new key she barely knows.

Also keep in mind that just because it's called Draco does not mean it will summon a Dragon equal in size and strength to the real thing. Most likely it'd be a person in a Dragon costume and weaker than the Zodiacs.