Its hard to say, in general there are a few meanings about.
1st. Is your Model out of focus?- Because of a warped bed/carriage/platform?
2nd. Different Material surface could cause not cutted lines.
3rd. Maybe its a known Marlin bug.
Share your gcode to see if the lines are closed and not open like your picture.
Here is something to read (the solution is in the last post):