Repeating pattern but different results?

So I have a chinese, no brand, gantry 8w diode GRBL laser. I’m currently engraving some wooden hearts all with the same design and using masking tape to give a crisp edge once removed and prevent any smudging.

However, different parts of the design seem to not be engraved on each piece. Here you can see the bottom of the date is missing on a few of them. The settings (500speed at 42%) is slow enough to give quite a deep engrave in the wood so I’m thinking it’s not the power settings but what else could it be? Thanks.