Bad Service from billing de[partment

On Wednesday 22 November 2017 i contacted the billing department informing them that my banking details have changed, i asked on numerous occasions whether the change would take place before my debit order which is the 15 of each month. On the 15th December i noticed that no monies were deducted from my account and contacted the billing department on Monday the 18th December. The lady in the billing department asked that i monitor my debit orders over the next few days. I now received and sms stating that i wont be covered for the month of December as my debit order did not go off. Firstly who is to blame, had i been correctly informed when i made contact with MiWay on the 22 November 2017 then my vehicle would of been insured, now i am currently uninsured, i am now considering cancelling my vehicle insurance with MIWay.