Call Centre unable to assist

For years I have had the same limit on my credit card and I often get calls from Nedbank asking whether I want to increase it and I always decline. The last few months I've been reaching my limit so I decided it will be easier to increase the limit rather than to keep paying small amounts during the month to keep my outstanding balance below the limit (every month I pay the statement amount in full). This morning I phoned to ask them to increase my limit by 20% of what it currently is (way less than what they've offered me in the past). I had to do a full application so I gave the gross salary that appears on my payslip, but I earn commission so my net salary is different every month and my net for the last 3 months was more than my gross so their system won't accept it. I can't give an average commission because it is different each month and I earn monthly, quarterly and annual commission so I wanted to do the application based only on my basic salary but their system won't allow that either! My salary gets paid into my Nedbank Current account every month and has for the last 14 years and they have my payment history but they still can't help me!