Sometimes it can be a tight hip/psoas holding you back from splits on one side since usually one side is tighter than the other. Try doing some psoas stretches (yoga has awesome psoas stretches), holding your splits longer, practicing oversplits, and other hip opening/hamstring stretches.
As for flexibility in general, consistency is key. Once a day minimum, but try for twice a day if you really want to loosen up for auditions. And give yourself plenty of time for stretch. Back when I was in dance I would stretch for an hour while watching TV!
Hope this helped! :)