If you are getting uneven burns some times skips letters. This is a problem I have experienced light wood, white paper or card. Some MDF have a similar property. It seems the only way is to pump up the power. The problem is now that the job is overburning.
A great little trick is to dampen the job with clear vinegar wait until is dry the see the results. I have the occassion to damp a job on white paper where the laser doesn;t even show trace and yet after the vinegar treatment the lines are well defined and iniformly dark.
I suppose its like the old invisble iink trick. Just experiment and see how low power your laser can still burn.