after getting ATC takeoff clearance I select Lnav. SOMETIMES, it engages, other times I have to wait until A/P is selected after takeoff. FMC is loaded, f/D is engaged. any suggestions ?

Here are the rules-LNAV engagement criteria on the ground:-origin runway in flight plan-active route entered in FMC-track of first leg within 5 degrees of runway heading-LNAV selected prior to TO/GA. Once LNAV is engaged, the LNAV switch light is extinguished until 400 feet AGL (PMDG NG LNAV stays illuminated)-LNAV guidance becomes active at 50 feet AGLLNAV automatically disconnects for the following reasons:-reaching end of active route-reaching a route discontinuity-intercepting a selected appraoch course in VOR LOC or APP modes-selecting HDG SEL-loss of capture criteriaTake care,John Floyd