FS2002 A.I aircraft don't lift of the runway???

Hi, :-)At EGCC Manchester in FS2002 my AI Aircraft taxi to the runway, ask for permission to take off then they get the clearance BUT ... the aircraft start's it's take off roll then at full speed it doesn't lift off the runway and just crashes into the grass at the end of the runway.Can anyone tell me why?Thanks,Regards Mike Brand.