Inaccurate payment allocation

In December 2014 I made a payment to Woolworths financial services for one of my loans. Two days later I phoned them to instruct the payment allocation, which they require clients to do. When I received my January statement, I realised that they had not acted out on my instructions and I placed a call to their call centre again. The call centre informed me that the problem is with their finance department. I told them that if this problem is not sorted out, I would reverse their debit order end of January and pay them according to my financial records. They still have not resolved my problem and it is now affecting my credit. This is not a problem I expected to encounter with such a big company. If the finance department is over-loaded with work that it takes longer than two months to resolve an issue why don't they employ more people. Otherwise train the staff so that allocations are done correctly. I received my end January statement yesterday and it still shows that my issue has not been resolved. This is completely unacceptable.