How to scrape bacteria from agar plate?

I have 180 plates with hundreds thousand of single and each differentially unique colonies. I need to pool all the bacteria. The problem is some of the plates are stored in 4c for around one week and when I try to scrape them off, they are like mucus and very hard to resuspend them in broth. Even vortex doesn't help much. I try to pellet down the bacteria but there is no distinct pellet formed, but layers of insoluble "mucus". So I ended up only take the bottom layer and suspend it and discard all the upper layers. My question is: will it be possible the bacteria from the "old plates" (those that form mucus) is not present in the final bacteria suspension? As they are all in the mucus layers which I have already discarded. I just want to make sure my final bacteria suspension contains ALL the single unique bacteria from ALL the plates. Even one single bacteria will do.