Structure is Weaker After Composite Optimisation

Dear Madam/Sir,

I followed the tutorials to perform a composite optimisation of a monocoque.

I believe that everything went fine until the discrete size optimisation, where I had to introduce manufacturing constraints. I did specify upper and lower bounds for each of the design variables, and then selected each ply and assigned 0.1 mm for carbon plies and 1.0 mm for core plies (TMANUF).

After performing another optimisation, the results were that all of the carbon plies had thickness of 0.1 mm and all the core plies had thickness of 1.0 mm. This results in a much weaker structure than the one I started with. Since then, I am changing the thickness of each ply by myself, I started with increasing the thicknesses of the core plies, but I assume that something went wrong.

Any help will be much appreciated.

Kind regards.