Coasters - inconsistent colour

Hi guys, wondering if you can shed any light on this.
I made a large order using the Xpres hardboard coasters. The prints were all fine (clean heads, same paper etc) press time/temp/pressure the same, but got a different result on some coasters. As you can see in the pic, the right coaster is correct and the left one looks much more desaturated in the browns.
I use an Epson L805 with Ink Experts inks and paper.

Is this possibly due to the coaster stock? It's the only thing i can think that's different.

Any help would be much appreciated

Attachment 7408

What heat press are you using? Just wondering if there may be temp issues.

I'm using the Adkins Magic Touch swing press.
I press 6 coasters at a time and can get 6 perfect ones and then all 6 will be the desaturated brown on the next batch.