renault megane scenic not starting?

Im having a huge problem with my scenic recently. its the dynamique 1.9dci import. trouble starting up, ive had the glow plugs changed, the timer relay changed, solenoids have been checked but still ive got a problem. For some reason the coil light isnt coming on all of the time on the dash board? is this a common fault somewhere? its id say 2 times out of 10 it wont start up.

Im wandering if anyone has had similar problems that could help me with this one as i mself and the mechanic that done the work are baffled. diagnostic tool isnt pisking up that theres a problem either.