VerticalScrollBar()->triggerAction(QAbstractSlider::SliderPageStepAdd) sometimes work and othertimes doesn't work

[ScienziatoBestia]

I use this code
@verticalScrollBar()->triggerAction(QAbstractSlider::SliderPageStepAdd);@
to reimplement the pagedown in a QTextBrowser.
It works always fine on desktop but on symbian it often doesn't work but after some singleLineStepAdd it restart to work.
Is it a bug? How I can solve?
Thank you