Regular user of o2 voucher here, have no probs whereever they are bought from.
Sounds like the voucher has gone through but your giffgaff account may not have logged it.
Contacting an Agent by using the link in the left hand column of you 'My giffgaff' page is where you can have your account queries looked into.
An Agent has the power to alter accounts, us mere mortals in the community can only offer advice.
Finding your way around giffgaff can be tricky as its not like other networks, but the savings in the long run are amazing.
Hope this helps you.