She Sits On The Sofa With Her Arms Folded Across Her Chest

Crossed arms are probably the best-known body language signal. They form a protective barrier, and, depending on the rigidity of the rest of the body, they say “I’m uptight” and “It’s going to take an awful lot to loosen me up.”

Annie seems to be telling you that she is nervous and anxious. Maybe her turn-on act in a company is just that, an act, and once she’s alone the real Annie surfaces. 

Or— what may be a lot worse for you—Annie may be bored with you alone.

Your best bet, if you really like Annie and you want to get her into a relaxed state, is to be gentle in your approach, and with your own body language show an
open, relaxed attitude. Keep your arms open and your body loose when you sit near her on the couch, or cross your legs toward her and form half of an intimate circle that includes her. If she likes you and can relax, she may unconsciously form the other half.

Crossed arms, of course, do not always mean resistance. Sometimes they’re a comfortable way to sit. But you stressed the fact that Annie only sits this way in your apartment, You must decide yourself whether it’s because Annie likes you too much that she becomes awkward with you once you’re alone, or whether she likes you so little that she becomes tense. In either case, your approach should
be slow, easy and relaxed. If Annie is so much fun with others, she’s surely worth a lot of time and patience.