It may be they were using a cheap janitorial shampoo which will build up over time and needs to be eventually extracted.
Polymer based encap products shouldn't do this so can be cleaned over and over again using this method.
The main problem with commercial premises is they are not maintained or cleaned properly. The Henry vacuum is wizzed over to pick up visible debris and that's that. Its worth trying to educate the client the importance of using decent vacuums and how to vacuum correctly.
I did this with a solicitors offices in Newcastle. They had called me in to clean the boardroom carpet which was a very good quality Wool Wilton. The pile was full to the brim with crap. I asked them when it was last vacuumed and they said it was done every day
I asked to see what they were using so they took me to the cleaners cupboard where there were 3 very forlorn Henry Vacs. You could have put them on your eyeball and they wouldn't have damaged it. On the back of this and a little educating I sold them 2 new Henrys and a Sebo BS36. I also cleaned all the carpets there for the next 10 years, mainly polyprop tiles, 20 year old which were always stained with curry and other takeaways as they often worked round the clock. Always cleaned up really well.
They eventually moved to a brand new office block. After a month they rang me to deal with some stains, curry again. To my horror the whole building was carpeted in 100% Wool carpet tiles in light Beige. The stains were permanent and they were shocked when I explained their error, or the Architects. I packed carpet cleaning in soon after so never got back. Would be interesting to see what state its in now.