It took my Rosco over a year to start purring very soft and my Monty also is so soft and hardly ever purrs.But I think it is just so soft to hear.It may be what they've been thru or just isn't relaxed enough or it could be that's how they were from birth.I wouldn't worry to much.Just make it known to her that she'll always have a happy home and maybe you'll hear her purr slightly if you put your ear close.That's what I have to do with Rosco and Monty.It may just be how they are anyway.