In Mail I can see the message preview but not the main message body.

Position your cursor on the rightmost edge of the Mail viewer window. When positioning your cursor over it, the normal selection cursor will change and bear a resemblance to this

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Drag that.

Drag that cursor to the left. There will be no visual confirmation of that action until you drag it more than halfway to the opposite edge of the window, or full screen if you are not using windowed mode.

If you prefer to use Mail's "Classic layout" the vertical column splitter becomes a horizontal one: drag up instead of left and remember you have to drag it a lot.

Is Mail running in windowed mode i.e. not full screen, and not in "split view" alongside another app?

Is Mail's View menu > Show Side Preview selected?

Post a screenshot if you think it would help. Refer to the Appendix in Writing an effective Apple Support Communities question. Obscure any information you don't want to appear in this publicly available website. That User Tip includes a link for How to edit a screenshot.

RichK wrote:

I am running in windowed mode and have Show Side Preview selected. I'm not sure a screen shot would show more than I already described. 

As a test, I tried moving one of my mail messages that didn't display the message body from the Cloud to my computer. I waited more than 5 minutes (whirling disk) and finally had to force-quit Mail. Is that telling us something?