It sounds like your fields are contained by the form not the tab - this can happen if, for example, you originally drew the controls on the form and then dragged them onto the tab. Try this:
1. select the fields that are "showing through"
2. right-click and choose 'Cut' from the pop-up menu
3. select the Tab control, ensuring the first tab has the focus
4. right-click and choose 'Paste', the fields should appear in the top-left of the tab control
5. click one of the other tabs and make sure the fields no longer show through.