I think its probably a total figment of your imagination.
Why would you feel that they dont Like you as much as the others??? What exactly do they do or say to make you feel this way?
I would continue to be yourself and just continue to be involved in sorority activities. After all, they chose you to be an AGD so they must love you. Its only nature that some people are going to bond closer together than others. Youve got ppl in houses who dont even necessarily like each other. But it doesnt mean that your any less important to the sisters then another. Who knows maybe you havent become close to the sister who'll be your best friend yet

Worst comes to worst and Im sure it wont, just ask them if they have a problem with you (i know its easier said then done).