I think I've found why I have the problem.
When my app is not zoomed, I can drill.
But when I have a zoom, even a few one, the "OK / Cancel" popup that is displayed is not displayed in the right place.
The user thinks the "OK" is at one place, and clik on it. But, in fact, the "OK" button is in another place, so the user don't click on it, but click elsewere and cancel his selection.
Proof : when I touch-down the "OK" popup, this one shouldn't disapear but wait that I "touch-up". In a zoomed area, a single touch will make the popup disparear.