Printing on 1/2" thick pvc

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Hello everyone, I'm new here and have a quick question on printing on 1/2" thick pvc sheet material (Komacel) I'm screening on two table tops for my saw table I manufacture and most of the screen works but after 3 or 4 times a few of my numbers won't print or little ink goes thru. I'm using Nazdar system 2 series vinyl black ink on white surface. Seems odd that everything else prints alright. Thanks for any help with this, Mel