VNAV is a path both vertically and with a speed consideration. The most common mistake is for guys to chase the magenta path indication using VS and then arrive at the decel point way above the VNAV path calculated speed. Re engaging VNAV at this point will cause the aircraft to fly level until with the FMC speed parameters, thus giving the impression that VNAV leaves you high. The ND clearly shows the green decel dots, the point at which the vertical rate will erode along with the speed to the speed schedule shown in the legs pages. Obviously if one fails to extend the lift devices, or take account of tail winds or anti ice then the issue is basic airmanship and lack of understanding of the systems. Equally, the messages " unable next altitude" or " bypass" should be an indication that somehow you have managed to overcook the approach, a quick check of the Legs Data will then show that according to your advanced planning you have asked the machine to arrive in two places at the same time , or loose 2000 feet and 40 Knots in 1 mile for example. The Systems manual navigation section has comprehensive descriptions of the process: for instance:
Normally, the target speed is economy speed above the airspeed restriction altitude and 240 knots below that altitude, until deceleration for approach. VNAV will not permit descent below the airspeed restriction altitude until the airspeed is at or below the restricted value plus ten knots. The start and end of the airport speed restriction deceleration segment is shown on the map as a green open circles with no labels.
The descent path assumes deceleration to reach the final approach fix (FAF), or the glideslope intercept point at VREF 40+20 knots.
Target speeds are changed by entries on the DESCENT page. Entries made on the LEGS page are “at or below” and may limit the target speed. Wind and thrust assumptions are changed on the DES FORECASTS page.
Deceleration points show on the map as green open circles with the label DECEL. Deceleration points show prior to:
• airspeed constrained waypoints
• holding patterns
• approach flap extension.
If more than one deceleration segment exists in the flight plan, only the next deceleration point shows. Deceleration points can also show prior to cruise holding patterns or other speed reductions.