I am having some issues and could use some help with fixing it. 

I have the P2, I am ornament designs out of 3mm acrylic. I have the settings down pat and have been using the machine for some time now with no issues. However lately I have been having an issue where I have multiple designs laid out on the acrylic. It goes through the cut on all the pieces but the cut starts to degrade as it goes from the left side to the right side of the sheet. The stuff on the left is cut perfectly as normal, but as it goes to the left the cut seems to be less strong. It starts to get to where its not fully cutting through in all parts and some it does not cut through all the way at all. I do tape the back of the acrylic with masking tape to prevent flash back. I am using a transparent green acryilic. I know it should cut thorugh with 100Power and 17 speed just fine i have had to bring the setting to 100 power and 15 speed and its still having trouble on the left side. 

I have check the mirrors, and have checkd the laser alignment. What else could it be? what else can I check? 

And yes the acrylic sheet is flat and all the same thickness.