Fed up pay on time every month, what's going on?

I am so fed up with Next on Line. I pay my account before it's due, I barely owe much, just over £100 now. I pay over min payment and yet every month they are reducing my credit to match my balance so I can't place further orders. I have not missed or been late with a single payment and on top of this I pay my daughters account who is at Uni.
Tonight they pretended there was a fault on the site when I tried to put things in my bag so I text my daughter who checked hers and it was fine. Basically they've blocked me from buying, why? I don't get it. I was intending to buy some school bits and pay with my debit card. They are rubbish, I pay all my bills every month and they have no good reason to do this. They haven't even told me why or asked to speak to me about my account, they just carry on treating me like a bad debtor.
Once I've paid it off I won't be returning as a customer, I prefer M&S for range and quality tbh.