What are the litres per minute & PSI of your machine. A 15LPM should be clearing most surface build up.
Are you using a flat surface cleaner then rinsing down with lance?
Are you lancing with turbo nozzle, flat nozzle?
Are you getting close enough?
I can honestly say I only ever use chemicals on graffiti and that is become less often with the heat of the H & M.
Rob