Tatum has done the same thing at the two Houston meet-ups we've gone too. I was afraid she was just stuck-up!
At home around our big Australian Shepherd, she's the alpha! She also plays with my husband's uncle's dog from next door - she's a Border Collie. I just chalk it up to the fact that she's so used to big dogs and seems to be scared of the small ones. She's done that at the ballpark too..... wants to play with the huge ones but is scared of the little ones. My advise is to just keep trying.