Tanks for your replies , i've tested by juste replacing the xtal in the schematic by a 27mhz one. The result is there is great oscillations but it's at 9mhz... it's the fundamental of the quartz i think, even it's a 27mhz in reality it's a 9mhz used at it third overtone. I've tried on a breadboard , maybe this can cause the crystal to not oscillate at the third overtone ? Or do i have to change the schematic before making a pcb prototype ?