It's hard to convert them, you could try a discounted offer for the first clean with a 100% guaranteed refund if they are not happy. Then you could end up with a customer for life.
If i feel i need to use hwe on low profile coomercial carpets, i always vac the hell out it first and i always bonnet after followed by air movers and i always say i will come back in a few days to check for wick back.
I'm always telling my sub-contactors/workers to pre vac on these jobs, till i'm blue in the face, but they just want a easy life.
I've got shot of a few because of this.
Most of them have never heard of bonneting or know how to work a rotary!!