Absolute positioning in Motif doesn't mix well with constraint-based positioning, as used by wxWindows to implement left-to-right, top-to-bottom layout.
If you know that all your widgets are going to be positioned and sized explicitly, you should pass the style flag wxABSOLUTE_POSITIONING to the panel constructor, in which case a Motif bulletin board widget will be used instead of a form widget.
NOTE: from wxWindows 1.60, the wxABSOLUTE_POSITIONING flag is obsolete, because all Motif positioning is now done this way.
One reason for a widget not appearing can be that the widget is sized just too big for the panel, and then Motif gets very confused. Try reducing the size of the widget.
Another possibility is to try setting the widget sizes first, and finally setting the panel size.
Widgets that are too close together may cause problems.
Some Motifs under some platforms appear to have very bad panel item problems in dialogs. Once cure seems to be to change the resize policy in src/x/wx_dialg.cpp: find where two lines are marked TROUBLE SPOT (from 1.61 beta 2), in the 'else' clause of the 'invisibleResize' condition. One line has XmRESIZE_ANY, the second has XmRESIZE_NONE.
Comment out or uncomment the first line, and uncomment or comment out the second line (whichever is not already the case). However, doing this seems to make a dialog box's size change dynamically when (for example) a listbox next to the dialog box edge is updated; so only change this if absolutely necessary.