This is usually due to a connection issue. Please wait a few minutes for your item to appear.
If the item still does not appear, please check that you have received a receipt for it and that it has not been subsequently cancelled. If you haven't received a receipt or you have received a cancellation, the payment will not be taken (though this sometimes takes a couple of days to become evident when paying by card).
If you have received a receipt and no cancellation, please do feel free to post in our bugs forum. Do note that we may advise you to contact Google Play in the case of store billing issues as all transactions take place via their servers.