Hi, Joshua.

When you finish your flight plan you check it on Plan mode? If it does not show a full route since airport but beggining at a certain waypoint, then it's not uncommon to the aircraft to be a little bit 'lost' when far from the FMC route.

It is supposed that pilot shall fly until near the waypoint and then activate LNAV.

Until there, the procedure should be, if you don't want to hand fly, at 1000 ft AGL to activate AP and HDG.

Another way to do is to activate Direct To the waypoint.

Regards

Schmid