possible causes for dark spots on tintypes

[starkphoto]

Hi all...

I'm experiencing something new that I haven't encountered before with wet plates. I am getting some dark spots in the highlights of my plates. The last time I shot wet plates, this didn't occur. I know it's probably chemistry related, but was wondering if someone could help me trouble shoot the cause? They don't really look like pinholes or comets to me, but I'm fairly new to this

Various Details:
DGX+ collodion mixed Sept 24 (I know, it's almost too old)
fresh silver bath (maybe 15 plates run through it)
fairly dust-free darkroom

[cuypers1807]

Those spots usually mean you didn’t wash the plate enough after fixing.

[MTGSeattle]

This was the first thread I saw with tintype in the title. I recently scored some old Apollo ferrotype plates among a big bundle of other paper, etc.. How hard are they to cut? they all have some rust, but being that 1 is either 20x24 or 16x20, there may be some decent plate area left. Similarly, there are 2 8x10 plates that could still yield a 5x7 or 4x5 if cutting them is easy. We work with a specialty metal flashing shop regularly; I wonder if their shear or break would be appropriate vs me using hand shears?