Sounds like you might have put your fields on the layout first and then put a Tab Control on top of them and then used "Send to Back" on the Tab Control, in which case all the fields will then be assigned to the front tab.
The correct way is to create the Tab Control first, then drag the first 10 fields to the first tab, then click the second tab and drag the other 10 fields onto that tab. Make sure all the fields are fully contained within each tab, i.e. make sure they're not extending beyond the edges of the Tab Control, not even by a single pixel.
Also, make sure that no part of the tab control touches or crosses the line between the header and body of your layout. You may need to click your tab control while in layout mode to select it and then press the down arrow once or twice to nudge it down and away from the header.
Thanks Nick and Phil for all the answers. It worked.