Problem processing payment

I have auto-renew turned on, so I was surprised this morning when I got an email stating that my service had been suspended because "your last payment didn't go through". So I head to the dashboard, and notice that auto-renew is off (it shouldn't be). I select my saved card ( a Barclays' credit card ) and press pay. It comes up with the verification window, so I go to my bank and I authorise the payment , and underneath in a small red box it says there was problem processing my payment.

Why?

I authorised it on the barclays app, there's planty of available credit on the card to pay £5.25 for my bill. Yet SMARTY says the payment failed. Again, why ?

You can't ever speak to anyone at SMARTY about these things. But it's concerning that a phone that I rely on could be cut off because of some issue with you being unable to take payments from my card. I have called my bank, and they agreed that I have authorised the payment today to SMARTY, but they see absolutely ZERO record of SMARTY even attempting to take a payment automatically before I made the manual transaction to renew.

This isn't the first time this is happening either. How can this be resolved?