What do you mean by blocked? They don't want to pay because they want to get them for free? Or is there some supply problem where sellers refuse to sell to them?
I think he means thing like the governments forcing producers to fulfill their contracts with them before exporting and fulfilling contracts elsewhere.
[1]https://www.ft.com/content/82fa8fb4-a867-4005-b6c2-a79969139...
[2]https://www.bbc.com/news/world-europe-56529868.amp