I had a similar experience with BT. In February I took out a mobile phone contract paying upfront cost, plus £65pm for my 12GB plan & £20pm for my son’s SIM 15GB plan. The offer advertised £100 Cashback, Plus £45 Amazon Voucher. I was told by BT I had missed the voucher by a few days (even though voucher offer started up a few days later) AND BT rejected my Cashback in June. I have put in a claim with Quidco, but still no success.
Again I feel as BT have acted very poorly, as I have met all the terms and conditions for the Cashback.
I have order number etc if BT would like to get in contact.