OS Window repainting

I am writing a MFC app that uses a separate SDL window to display
graphics. Everything seems to be working OK, but the SDL window does
not repaint unless some other Window moves over the top. For example:

I blit images to the screen surface and nothing shows up. I grab the
title bar and drag the window off screen and then back on (never
letting up on the mouse) and the areas of the window that come back on
screen have the image.

I remember reading something in the docs about a repaint message, but
now I can’t find it.

How can I force the window to be repainted (not my blitting, but the
OS repainting the window). Me blitting to the window does not force
it to visibly update.