Dishonest charges

As a cardholder on a 6 months interest-free account, I find the service frustratingly poor. Every time I receive my account, my charges are on a 12 month interest bearing account and need to get in touch with the complaints department who responds days later to say my charges are transferred to the 6 months account without reversing the interest. Logically, this recurring error, where you never get to speak with 1 person to get the actual issue resolved, the interest remains payable. Aside from being an absolute monthly time waster, this is starting to look like a dishonest ploy to keep on charging interest!!!