Same thing happened to me as well.
I have been upgrading my iPhone every year for the last 3 years and used the Barclays monthly payment option for all these three years. However, out of the blue now I have been declined for the new upgrade.
I have had a mortgage for 2 years, (so nothing new), always paid on time.
I have had the same credit cards and been offered a higher limit even, and always pay the full amount back every month.
No other loans, and I have an excellent credit score (I can see that they have done the search on Experian too). My salary has also significantly increased since the last time I applied for an upgrade.
So go figure..
I've spoken to Barclays and they said they cannot give me the specifics, they just think I don't meet the criteria (refused to inform me on which criteria).
Oh and the mortgage is with Barclays too!