Inconsistent results

Pretty new to sublimation, but had some good results with what I've been doing so far. One thing that I have noticed though, is if I'm doing a batch of products I get inconsistent results (with the same substrate and print, which was outsourced to someone with a SG1000).

Any suggestions on how to make sure of consistency of output? Or what would be the reason for the difference in black transfer between each product?

Thanks!