Surface disappears when join

Hello, there. So I do not have much experience, I am only using the trial version for my classes. I created some sort of object from many, many surfaces and when I try to join all of them, the front surface simply disappears. I also tried to use the boolean union as I am trying to make a single object when clicking on it, but of course, boolean union failed. What can I do to make it a single object?

I have also tried to explode the surfaces and to join them again, but the front surface disappears again. Any advice please?2

without the file it’s hard to find the correct answer but I guess that the problem can be two o more ellipses touching each other, causing manifold edges over the front surface.
when this happen, the surface can not generate the mesh representation correctly.