Hi @Simone_Brandi,
I’ve had the same problem and I did the same as you without positive results… Check this topic and it might help you on your way: Is this forum going the same way as Makerbots?
It’s definitely not the plate.
Like me you lowered the build platform to solve the issue. I ended up needing to raise the build platform for the parts to stick. I know this doesn’t make sense… But you’ll find more info in the link I posted.