kivy button on_press event misplaced on hp windows 8 touch tablet

[alessandro bonvicini]

HI all, 

i am totally new to kivy ,so please don't blame. 

I tried a simple example ,  a column and a row  of buttons. ON windows 8.1 with mouse it works well, as expected: every button receive and manage his own on_press. BUT when I place the code on a hp window 8 hp 900 touch tablet with the SAME python installation and version the app exibits a strange behaviour.

 The buttons that reacts  to the on_press event is not that one i press BUT the next beside, on the left. And so is , of course, for the callback. THe callback invoked is that of the nearest button, not that one binded to the button i press. 

Like if the sensitive surface of the button would be shifted to the right of a certain number of pixel.

No one error , nor suspicious log message...only like if the touchscreen would not calibrated....

But obviousliy the tablet works well out of the kivy app.

What can i do to correct this behaviour ? Is there something that can be done ? No one had this problems?

THank a lot in advance

Alessandro