I have the same issues with my girls. I am not sure how old they are (the people at PetCo could not tell me) but i have had them for 8 months now, and play with them for atleast an hour most days. Mocha is very timid and only comes out if i have food like your girl. My other, Peppers, is much more curious but will not stay still for her life. I eventually just bore them while we hang out on the couch or on the floor in the bedroom and they just cuddle under blankets i have spread around. Most people would say it just takes time, but i think it may be part of thier disposition. The fact that your other girl gave you an instant connection sounds pretty remarkable! She was special, and it just may be hard having other rats that are different with such a great expectation. I am hoping mine will sweeten up, but I have a feeling that Mocha, my timid one, will always be the same since she hasnt changed AT ALL since i have had her! I love her anyway, and it tends to balance out with her sister 
