Maybe she's been busy and hasn't had time to reply, or there's a problem with her computer - maybe she didn't get your email or even your letter.
I see why you're feeling like that though
I know you're not going to want to do this - but just in case there is a good reason why you haven't heard back from her. Why not try and call her - you could make it seem like you're not really bothered and were just checking before looking for someone else to see.
That way you'll know for sure.
Take Care
