[Misiur]

Interesting issue. Long story short, you have very specific conditions for loop to go further, but you select items randomly until all blocks are used. So, with each iteration of outer loop, you have lesser and lesser chance of actually hitting those conditions.

https://gist.github.com/Misiur/99d23545ebdefdde4f57

I didn't touch logic of your code, only that loop issue. If you have any questions regarding what the hell am I doing with those iterators, ask

Cool script!