Experienced & qualified but always gets offered dishwashers wages?

Have qualifications and 5 years experience and have own means of production, wages nationally for my trade tops 30k to 40k but I always get offered outrageously low wages, such as �12k - as a result, I've been doing menial work for the last 4 years because I can get more money sweeping up than doing a full engineering design job. I'm starting to think something is drastically wrong...is it my race, or my accent or my name or what...???