Strange mouse behaviour

[juagargi]

Hi all,
using grabMouse(Qt::BlankCursor) from the main window I expect the mouse cursor to disappear. Sometimes, and for a reason that I can't explain yet, it does not, when in debug mode. Qt 4.8.4, Visual Studio 10, 64bit.
To make it weirder, we've seen sometimes the method mouseMoveEvent(QMouseEvent*) not being called as well (after calling grabMouse). In this case, the application is compiled in release mode.
I've actually tried to debug the first problem, about the grabMouse, and I don't see any reason why this should not work. Any hints on how to proceed now?
Many thanks in advance,

Juan A.