Superglaze mug issue

[Rachaelx777]

I have purchased white heart handle mugs from https://www.dyesublimationsupplies.co.uk for the first time as the usual supplier which are orca coated aren't in stock. I'm finding that the black is not looking good, almost a greyish and doesn't pop. Other colours are ok. I have tested in the press and oven on usual settings that work fine with orca mugs and it's noticeably worse. The same print is perfect on the other orca mugs so not paper/press/ink. Just wondering if anyone else has used this brand and had any issues? I've contacted dye sub supplies about it but not sure if they're just not as good quality as what I normally use as I had a similar problem when trying signzworld mugs once...