OK, so I tried my hand at my first SMD design since JLCPCB has the assembly option. The process of selecting the SMD side to be assembled, as well as the components went easy. When I got to the quote screen, my Gerber view of the bottom layer looks like this:  However, after the components are selected, most of the traces are gone and it looks like this:  On the top layer, no traces exist any more. What can be the problem?