question about fuel tanks

Hi all ,last night i did a flight from schiphol to heatrow. Brought two wings full of fuel . The left wing was empty just moments before establishing ... but , the plane was not switching to the fuel in my right wing ...So ... engine went dead , stalling was following shortly and me and my passengers did not make it home... sad evening.What happend here. i flew 737 controlled by pm systems and pm glass cockpit. , what i did do before this happened was, i tried to level the fuel in the wings before landing , so i switched the crossfeed switch , but nothing hapend. ... . why did the fuel in the right wing not flow .. what could cause this , pump switches where on , no red light what so ever....i gues i did forget something on the pm systems overhead but what ?i hope to find out what went wrong here ,thnx ..