Hi Kevin,
She may have moved on.
There is a chance that she may have liked you in the beginning.
This may be why she would smile at you often at Sunday school and look at you from the corner of her eye.
However, you didn’t ask her out.
By the time you sent her a friend request on Facebook, she may not have accepted it because your window of opportunity with her had passed.
When you take so long to make a move on asking a girl out, even if she may have liked you initially, she will ultimately stop liking you.
In other words, you don’t have all the time in the world.
With this particular girl, she may have just simply run out of patience and moved on.