I see this characteristic a lot.
The example image shows a series of lines all at the same power setting. Except the shorter ones are darker, as are the beginnings of the longer ones.
When cutting, the power meter jumps past the desired setting when coming on, but I figure this is because it's an analog meter and the power it's measuring is coming up "instantly".
The only thing I can figure is run the shorter lines at a lessor power setting. The one "missing" line was run at a lower power setting with a bunch of the short lines and they are somewhat visible where as the longer line is not visible except where it begins, it's a little darker.
The "missing" line is with a power setting of .032 VDC input is the first at which I get output from the laser that does anything. Setting it to .033 VDC input gives me the lines that are plainly visible.