Alan,as mentioned above :

,it tends to be most carpets,i.e. wool,80/20 mix,synthetics.Average room vacuumed for 20/25min prior to pre-spray as per instruction from Mr Paul Pearce to remove 79% of dry soiling,so fibres should be removed during the vacuuming stage.I perform fibre and colour tests on initial survey as per IICRC training.
My machine is an Alltec Advantage I use both vacuums to extract could this be causing the problem?
Barbara