You must have erred when placing the grid on the page control. For each
tab of the page control, there is a TTabSheet object. It's the tab sheet
that needs to be the parent of your grid. You placed the button and the
combo box on the sheet, but perhaps you placed the grid on the form
itself, and because of Z-order it only _appears_ to be on the page control.
Cut the grid (Ctrl+X), switch to the tab you want, and then click
somwhere in the middle of the page control. The resize handles should be
positioned around the inside of the page, indicating that the TTabSheet
is selected and not the TPageControl. Paste the grid there (Ctrl+V).
Another way of selecting the tab sheet (besides clicking on it) is to
find it in the combo box at the top of the Object Inspector. If you have
the right Delphi version, you can also select it via the Structure View.