my biggest pet peeve is a dirty wand and machines ,,,, van tooI get guys in for wand repair on a daily basis. And the first thing I do every time is, I flip the wand over and look at the jetted side. Usually it's clogged and gross with hair, grease, debris, etc all over it.
I show it to them and ask "How do you expect to get a clean carpet from a dirty wand?" If there's a ton of pet hair on it, I remind them that flea eggs can piggyback from one house to another on a wand-head, and they don't want to be responsible for cross-contaminating a non-pet-owner's home with someone else's cooties.
Of the portables and TMs we see for repair, the ones that almost always have the most catastrophic failures also tend to be the ones that aren't kept clean and maintained. If it's full of dirty rags and fast food trash, and it smells like sewage, it's a lot less likely the machine is getting the attention it needs to keep it running smoothly.