problem while calling DrawNow().

my first guess is that you have declared it a bit wrong, so you have in fact not overloaded the draw function, but created a new one.

make sure it really is

void Draw(const TRect &aRect) const;

and nothing else.
Its very common to forget the last "const"

Have you called SetParent() on the CCoeControl, specifying some other control as parent?
If you have, you must make sure your control also is a child of that control through CountComponentControls and ComponentControl calls

If you havn't used SetParent, the only way I know of that could make DrawNow fail to call Draw is a wrongly defined Draw function.
And I'm quite familiar with the inner workings of CCoeControls...

That is, if you call DrawNow directly on the control you want to redraw.

If you are calling it on some parent control of the control you want to redraw, then you must make sure that the framework can find its way to the control through CountComponentControls and ComponentControl calls.