LNAV engages when laterally within 3 nautical miles of the active route leg. If outside of 3 nautical miles of the active route leg, LNAV engages if on an intercept heading of 90 degrees or less and the intercept will occur before the active waypoint.
Remember that when you press LNAV during a turn, it will use the track when pressed to intercept, thus will roll out of the turn to establish in the track you passed when pressing LNAV instead of completing the turn... Ensure when pressing LNAV you are wings level to avoid the roll back onto track before intercept.