I used the touchscreen from a tablet (odys space : yeah really)
This was a capacitive focaltech touchscreen with an i2c interface (twowire)
The first time I used the touchscreen some areas where not sensitive...
After searching the net, it appeared to be a firmware problem
I had a look at the touchscreen driver, and I'm sure it automatically updates the firmware!
(so even if you have a working touchscreen, it could not be working after loading the module...)
So here is my procedure to install another firmware on a focaltech touchscreen :