qtcreator custom plugin inherited by a QwtPlot

Hi,
I'm trying to create a qtcreator custom plugin inherited by a QwtPlot.
I already did several custom plugin starting from a standard qt elements.

The Plugin works fine under linux, but I cannot add custom item into the plugin context menu. To solve this I derived my plugin from a QFrame and inside it I put the QwtPot widget. This trick works fine.

The trouble is that building the same plugin under windows doesn't work: when I drag the plugin into the designer form, I see only a normal QwtPlot; into the property list I see only the Qwt property and the custom property are listed into the Dynamic Property section as an unrecognized plugin.
If I comment out the QwtPlot creation, the plugin works fine (Property and custom item into context menu), obviously without the QwPlot widget!

On the Linux machine the Qt creator is based on Qt 4.7.4 (32 bit) and the qmake version is 2.01a
using Qt version 4.6.0
On the Windows machine Based on Qt 4.8.5 (GCC 4.8.1, 32 bit) and the qmake version is 2.01a
using Qt version 4.8.5

If you need more info or the code itself, please ask!

Can you help me?