I left a lot of my prized possessions when we moved to Guatemala. Didn't even store them, just sold or gave away. But one thing I saved and brought with me was my yogurt maker and strainer. And am I glad I did, because the Greek yogurt I love isn't available and any other kind is outrageously expensive.
The milk I use is boxed. At first the yogurt wasn't turning out. It didn't get thick and there was too much whey left when I strained it. I finally discovered the problem was the milk. I wasn't checking the label and had bought milk mixed with other ingredients-- sugar, water, vegetable oil, etc. As soon as I corrected the problem and bought only leche entero, or "entirely milk and nothing else" the problem was solved.
It had never occurred to me that what was labeled as milk might have other ingredients added to stretch the product!
(For more details on making homemade Greek yogurt, see my website.)