When doing it via mobile it tends to have issues. Did you wait for the green bar to fill and a message saying (or something along the lines of): 'Uw transactie is voltooid!', inside of the filled up green bar? Because if you did not wait until that showed up, or did not click the bar when that message were to pop up, then it can be one of two things:
a) It is taking a long time for the server to complete the transaction
or
b) by not clicking the green bar or waiting for it to complete, you technically voided the payment after the money had been taken.
If it is a), then just wait a bit. If it is b), then I can am not sure what can be done about it.