Why doesn't my machine make good quality foam?

Similar to the crema on quality espresso, this "foam" is an indicator of the freshness of your coffee. If it's thick and creamy white, it's very fresh. If it's present but thin and watery, it's less fresh.
https://www.quora.com/What-is-the-foam-on-coffee-made-of-that-appears-on-milk-based-and-even-black-coffees
The foam found at the top of your cup is "crema" comes from the espresso making process. Espresso is prepared in a metal contraption, which you pound down and hot water is pumped through at great pressures. The pressure pushes out the little oils from the coffee beans into the liquid. https://www.joyridecoffeedistributors.com/blog/coffee-news/where-does-the-foam-come-from

get a quality carboard box slightly bigger than the unit
wrap the unit in plastic
place a later of foam chips in tye bottom of the box
put in the unit and fill allaround and over the top with the foam chips
seal off with duct tape
Foam chips or the like can be got from removalists
a lesser product is to wrap securely in bubble wrap before placing in the card board box

Remove spark plug and and ensure there is actually a spark when the machine is cranked.
Ensure air filter is clean. Foam elements can be washed with hot water and detergent, squeezed dry in a cloth and re-oiled with a teaspoon of engine oil spread through the foam. Paper elements can be cleaned with compressed air blown from the inside of the filter but ideally should be replaced.
Check fuel lines aren't kinked or blocked restricting fuel flow.
Ensure fuel filter isn't dirty. This is probably located at the end of the fuel line in the tank.
Remove the air filter and check the choke plate closes ok when choke is turned on.
Air locks in the fuel lines will also cause this symptom. These can be caused by running the machine with low fuel level in the tank and cutting upside down or at a sharp angle.
To remedy this keep pressing the primer button i present until it feels spongy and fills with fuel.

They put it there to help newbies make foam, it is perfectly ok to use without.

Our aeroccino worked fine for 2 years and suddenly it didn't made foam anymore. We tried the "inverting trick" without much success. We contacted Nespresso and they suggested switching from regular fresh milk (we used 2%) to UHT milk (harder to find in US) and to use it cold and freshly opened. So we tried that and it works.
Hope this solution works for you too