So today I'm flying from KSAN to KSFO.  Things are looking good.  I'm step climbing holding the ALT with the hold when I get to each waypoint.  The VNAV is working well not overshooting the altitudes so I don't need to use the ALT HOLD much on ascend.  I get a T/D and I start to descend again holding the altitude with the ALT button at each step.  Then I resume VNAV at 20Kft but the bird does not want to descend.  I'm not sure why.  So now I have to take over with the ALT hold with the V/S and flying becomes a chore.  So I'm not sure why the VNAV stopped descending.  Was it because I was holding it at each step?  Did it forget what it was doing?  Do I need to just let it be?  Help! :-/