Without looking at the actual model, the best we can do is guess. My guess is that the four corners are not in plane. Drawing a line across the four sided space will close the opening with two triangles.
C-Class Rear.skp (286.2 KB)
Im just not sure why it would do that with a few and not the rest. I created an array and copy/rotated it 27 times for the amount of teeth.
The model is too small and is tripping SketchUp’s nearby-vertex cleanup operation. Scale up to a larger size before adding the geometry and then scale back down when you are finished.
When trying to make a sketchup model to scale, do you just have to double every dimension and then scale back 50%? Im still unsure about how the whole scaling thing works when you are trying to make something to the dimensions.
If the thing you are modeling is small, working at a larger scale helps prevent problems. The common advice it to work at a scale factor that is some multiple of 10 larger. That makes it easier to work with dimensions because it’s generally just moving the decimal point. If you like multiplying by 2 and that makes the model large enough, go for it.
This model still allowed me to fill in the missing faces, although shortest edges are indeed small.
OP should have aligned all 27 segmented circles and teeth better to avoid creating extra (hidden) edges. See this closeup: