I jsut got an "eVoucher" for £10 and added i online to my Vodafone account. It says my calling balance is now £11.00 however my data allowence is under 500MB. How come it hasn't increased since I added money?
This makes it much easier to get to the bottom of things!
I couldn't be 100% sure before but with the mention of £15, I'm assuming you're using a Top Up And Go dongle rather than a PAYG phone.
The minimum top up for a dongle is £15, so if you credit less than that, the system cannot recognise it and convert it to data. You need to top the credit up to £15 and then contact the Pay As You Go team top arrange for them to convert the separate amounts into a £15 data top-up - the system won't recognise the total credit and do it automatically.
It's also possible that if you pop into your nearest store that they can convert the credit or put you in touch with the PAYG team directly to arrange this, so don't worry if an email is out of the question at this time!