My guess, there are double lines.
I would probably recreate this Geometry and have it fully defined.
Use the Repair Sketch option on the sketch toolbar and you can check for gaps smaller than a specific size then use the magnifying glass to zoom in to the problem area. (In addition to Sarah's comment above).
In your case the hexagon has a gap at the top and bottom centre where the two line endpoints meet. You can click both endpoints then use the merge option to connect them easily.
Another way to check you don't have gaps in closed contours is to turn on the 'Shaded Sketch Contours' at the top of the sketch toolbar which shades in a closed profile if it has no gaps! This is a visual way to check you are creating geometry in the correct way.
The fixed part is attached.