I too had fun and games trying to top-up my 4GEE PAYG dongle yesterday. My debit card was also declined. My bank blocked the card not once but twice and it took me about 2 hours to get it sorted!!!
I tried using a credit card and that went though first time straight away. Try a credit card if you have one, if not try using the top-up card in a supermarket etc your PAYG SIM card should have come with.
EE blamed the bank, the bank blamed EE saying they are seeing a lot of fraud from EE top-up transactions at the minute.