Whenever there is a perceived loss of power, it could be caused by dirty or damaged optics, bad laser beam alignment or a focus or table level issue. Below you can find links to instructions on checking and leveling the table as well as checking and fixing the alignment of the laser beam, and how to properly check the optics in the machine.
First, check the optics for any damage, including burn marks, pock marks, scratches, cracks, flaking of the layers on the mirror or discoloration of any kind. To adequately check these, you will need to remove the lens carriage (lens and mirror) from the machine so that you can check the mirror above the lens.
If you look between the lens and mirror to the back plate that the assembly rides left and right on, you will see two small Allen Head Screws. Remove these screws and the whole carriage assembly will come off. Now you can examine the mirror above the lens.