Is there something wrong with my guinea pig?

Ive got a pair of female guinea pigs they get on fine. even tho theres a age gap. we got out eldest piggy back in october 09' as a single and after months of hunting we managed to find our baby. the big girl didnt really talk or walk or do anything when we first got her so we thought when we get another one she'd be a bit more lively, trouble is shes still the same she doesnt really do anything shes quite happy to sit in the same spot all day. or sit in her igloo allday. we got her as rescue so we dont really know her past but she isnt chatty like our baby she doesnt like to waddle round when we put her on the floor she'd rather just sit there. has anyone experiance anything like this before? and have they got any ideas?