Payment calculation

Hi I’ve just started a new job and my wage has gone up soni understand my payments will before this job I was working for lower wages and had to revert to using my credit cards to get by. So now I’m trying to work extra shifts as much as possible to cover this cost after my csa has come out. But they keep putting my payments up based on my over time I’ve tried to explain why I’m working 6 days a week but there not interested how can’t they don’t calculate from my basic income?