Help with random procedure.

Hello everyone. 

I try to make a random procedure that triggers when the player destroys a block. 

I can do a random probability with a single item. However, when I try with multiples items, nothing happens when the block is destroyed. (First image.)

I also tried before with another type of procedure. It worked, but not the way I intended. When two items share the same value, they spawn together. I thought limiting the number between two values would avoid that, but unfortunately it seems that it does not work. (Second image.)

Sorry for my English, it's not my native language.