It's a combination of adolescence plus the dog being put into situations where she could choose not to come--so she found out that it's optional. Oops.
One thing to try is do not call her to you for 6 weeks unless you have a line on her. Practice recalls with the line, but only a few a day. Six weeks isn't so long, when you think about the length of a dog's life, and how much this behavior can mean.
Keep the line loose, so that she isn't getting directions from it. After all, you don't want her to expect to "feel" the restraint forever. If you do have to tweak her in to you because she tests the theory of not coming when you call, use the line, but get it back loose quickly.
Praise, praise, praise and reward recalls, even though you are guaranteeing them! And do that forever. Always make a dog glad to come to you. All punishments are eliminated at obedience to the come command!
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Kathy Diamond Davis, author, "Therapy Dogs: Training Your Dog to Reach Others," 2nd edition, and the free Canine Behavior Series articles at http://www.veterinarypartner.com/Content.plx?P=SRC&S=1&SourceID=47