I know that might sound obvious but in December I took advantage of the price match to Tescos £10 sim offer and upgraded my wifes phone at the same time. Both plans should have been receiving a £5 discount making them £10 and £15 respectively, however neither were having the discount applied to them.
When I called today to sort out, after initially having to endure the overseas numpties I got put through to a call centre in Teeside. I spoke to a helpful and friendly lady who said that they have been getting loads of calls over the last few weeks due to peoples discounts not kicking in, apparently there is a 'glitch' in the system for people who's discount codes end CA I think she said.
The only way she could sort out was to put credit on my account equal to the discount for the next 6 months, and told me to check back then as they should have sorted the glitch out by then.