In the case of this model there are two open gaps as indicated by the picture below. I would rollback and correct your 3dsketch for the particular surface that is not meeting to close the gap then rebuild and then the knit (with "try to make solid" option enabled) should be able to make solid.
I find the best way to diagnost problems like this is to turn my background to white, menu>view>hide all types and then go to wireframe view in your heads up view toolbar and then look for blue edges verses black edges - and entirely black edge model means that it is fully water tight and should be able to be made solid.
Edit: Also notice after producing the image that the mirror seam is not matching from left to right.
You can also use the menu>tools>check feature will also identify open surfaces.
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