Thank you for your post.
When you click on a tab, only those fields attached to the tab will display. If a field was added to the first tab and you click the second tab, the field attached to the first tab will not display.
If you feel a field should be showing, go into Layout mode and place the field in the appropriate tab so it gets attached properly. Then go to Browse mode, click on the appropriate Tab and see if the field now appears.
If you originally had a field outside the tab, but then used Inspector positioning to place the field on top of the tab, the field will be unattached. You must manually attach the field first, and then use inspector positioning for finer positioning.
Let me know if you need additional clarification or continue to run into difficulty.