NBT Tags and structures.

Hello, sorry to bother you again. 

Here is my problem : I have made a block that is replaced every three minutes by another. From here, everything is fine. 

But, I wanted that block to be a part of a structure that spawn randomly into the world. However, I suspect that the blocks are not longer considered as "blocks" but as a structure. So, my NBT tags no longer work. 

I tried to place manually the block side to side with the same block generated as a structure. In the first case, the block is replaced. Not in the second case.