Adwords Credis but Bank Still Charged! :(

Hi everyone. I spend between £500 and £1000 per day on Google ads. I went into billing and saw that I have a 'Credit' of -£6700. My payment threshold in £500. My bank just took a £500 payment and my credit went up an extra £500 to credit of -£7200.

Why am I being charged to my bank when I have all this lovely credit available. Isnt Google supposed to use my credit before charging my bank card? If so how can I use up my credit before making any more payments? Thanks