I have had such problemes but it has always been something. The lenses are the most usual now. The slightest bit of crud or even smoke being too near one of those windows will screw up the next cut, and it will take an amazingly small amount to do that.
Before I figured out the problems with magnets they were the problem most often. It was extremely frustrating not finding the issues the magnets were creating when I was not aware they could be the problem, eventually wrecking my machine almost at the end of the warranty period.
In order to know what settings will work, I make these and it will show perhaps one setting that barely makes it through and the settings that make it through easier and easier. If I am worried that it is very important for the cut to be clean I might take the second or third one that makes it through. In every case, the outer window of the head is the most prone to collecting a film and cleaning that makes a huge difference in the cleanliness of the cut even if it is a bit overpowered.
As others have noted the material can be a big problem even different places in the same material. The worst I have seen was 1/4 inch birch plywood when even extreme overpowering was no help.
The two 1/4" plywoods I have had the best results with are one called Revolution and the other is Oak plywood though that one needs to be catrfully chosen for fewer knots on the bottom.