Why didn't my friend follow me back on Instagram?

[Sunshine on a Cloudy Day]

So I have a friend in my class who I've known and talked to for a while.

I gave her my number & allowed her to borrow my copy of our class' required course pack over this past weekend.

She herself asked me for both.

I showed that I found her on Instagram from the Contacts area and she gave me permission to follow her and accepted me but she did not follow me back!!

I know its just Instagram but I feel a bit weird about this if she and I are gonna be friends.

Why might've she not followed me back?
And if you're gonna tell me to mention it to her, then how??

[Anna]

Check if you are accidentally blocking her. Happens to me all the time!!

[momentum deferred]

I sometimes wait (on facebook) to friend someone who has friend requested until they send me a message, because I don't want to be friends with someone who doesn't want to be my friend in person, just to add me to their collection of people they barely know but call friends on facebook

I'm not saying you collect friends but she might do what I do