I have grown houseplants all my adult life with great results. In the last 15 years or so, I've lived in places which have the newer UV blocking window glass, and since then almost nothing grows happily on my windowsills.

I firmly believe it is because if of those windows. I have researched this several times over recent years, and not many support my "theory", especially professionals in the window business.
Has anyone here ever wondered about this?

I was told that windows block enough UV rays to not be beneficial for my parrots. IDK how much UVA-B rays plants need, but I do know my window sill plants do still benefit from the light. That is why my plants that are brought in to winter over, do well enough when sitting in front of a sunny window. Our windows are 20 years old. IDK if that makes a difference.