Why isn’t the engraving uniform?

Assuming:
All the missing pieces and jagged lines are intentional
The material is acrylic
Scanning direction parallel to the X axis

Then:
New batch of acrylic with sub-standard quality control
Worn / loose X axis belt
Loose mirror
Loose focus lens

More details on the speed / power / machine will be helpful, but it’s almost certainly a mechanical or material problem rather than a setting / configuration / software issue.