Issues creating lithophanes need help

I am using 1/4" acrylic for my lithophanes and have done 8 so far using this material. The last 3 have had streaks where it seems its didnt cut deep enough it looks like a roughing pass. Can anyone tell me what is causing this? It is getting worst with every carving.

I’m using Aspire, feed rate is 2" per sec and plunge rate is 1" per sec

It looks like you might be loosing steps on the Z axis. Have you checked to be sure the z belt is tight and that the pulley is not slipping on the shaft (grub screws are in nice and tight). You can also try turning the Z by hand and be sure it is not sticking anywhere.