I have sometimes this problems, just check all the waypoints, some times a waypoint was incorrect or misplaced by the FMC, what i do to check that is to place the nav display on plan , fully zoomed out and the in the FMC legs page i click on step to check for a logic flight plan, as i plan using sky-vector, i know how to route should look. The problem is that, if the fmc tells there is not enough fuel, the AP wont engage.