could be your washmitt - is it very old? are you using the two bucket method (one bucket of soapy water, one of plain water to rinse the dirt from the mitt after each panel or two). foam lances are designed for pressure washers really. you can use one of these;

http://www.autojoy.co.uk/hose-end-sp...495-p-376.html

on a normal hose, foam won't be as thick but it'll help with dirt removal