you might also want to check that you have a ground actually there, as from what you said "..where the ground would be.." that would indicate you don't actually have a ground, so from display(pull down) goto display guides and check that ground is checked, shadows will appear with or without a ground in preview mode,unless you specify to not show shadows, but when you render they wont be there unless you have display ground checked, you might wanna texture the ground too.