Hey Giancarlo
It's tough to tell for sure from this angle, but it looks like you're set up too close to the bar. Back up half an inch from the bar. This will get the hips in the right place, so you can set the back properly. You're not really getting the back very tight to start. Get the chest up hard while you're holding on to the bar and pull all of the slack out of your arms and the bar. Then, when you've done that, break the bar off the ground by pushing it away with your feet and extending the knees. Right now you're yanking it off the ground by lifting the chest to open the back angle, and your quads aren't helping at all.
Setting up further from the bar should help to start.
Good luck!