Hi A.!
Just keep doing what you are doing and be consistant. The more consistant you are, the more likely they are to pick up the signs and sign them back. Most babies don't start signing back until they are 8-13 months of age. Also, if they are working on a large developmental milestone (like crawling, cutting a tooth, etc) the signing back may drop off or disappear totally, but don't worry - it will come back.
We started signing with our daughter at about 8 months and it took her until she was about 14 months before she really started signing back and then she added about 1 a week for a while. It slowed down for a while and now she is adding signs and words like crazy!
I teach parent workshops and parent/child play classes if you are interested. I also sell a series of DVD's specifically made for children under age 3 to help them learn the signs. My daugther absolutely loves them! It was really interesting watching them with her before she could sign. She would concentrate really hard on them and you could see her hands moving like she was trying to figure out how those babies were doing it. It was really cool! You could almost see the wheels turning in her mind!
Please feel free to contact me directly. I would love to talk to you further!
J.