Image is not lasered constant

Hi there,
I want to engrave an Image on 1mm thick card board.
I have done this before, but within the last trials the quality got worse and worse (from pic 1 to 4) .
You have any suggestions to me, what to try?

In the past my laser module got broken two times. But I am only running on at least 80% power and hadn’t any trouble for such a long time.

Greetings
Björn