The problem is most likely incorrect wall/level heights.

Set the level height of level 0 to the same height as your walls on level 0 (right click on the level tab to edit level properties).
Check the floor thickness you have on level 1. Level 1 elevation must start at the height of level 0 + the floor thickness.

Example:
level 0: walls 250 high, set level height 250
level 1: walls 250 high and floor thickness 20: level 1 elevation 250+20=270, level height 250.