I had a similar prob,
Cleaned a custys carpet and was finished,so I was showing her how upholstery tool sprays and extracts,but because solution tank was empty nothing was coming out,have you actually been cleaning my carpet she says.Totally trashed and stained white carpet,as good as new after 3 hours.Refilled tank pump running but still no solution coming out,even when I pushed nipple of hose, felt a right pratt as I thought my pump was gone,and still had a job to go too.Got home to get spare machine,but decided to try pump again,hey presto,working again.I assume it was just an airlock,or because I had totally emptied tank ,it needed extra priming to get solution to tool.This has happened twice.I think the pump is an on demand pump.
Sam.