A lot of it depends on the material. Not all plywood is created equal.
If you are getting a lot of char, and the cut still isn’t making it through, it could be something in the glue that they use for the plywood or bondo, or air pockets inside. Any of the above will keep the laser from cutting it.
18 passes sounds like air pockets. There’s a primer on how to check your wood with a flashlight here: Trick for inspecting plywood