Why won't the PMDG 747 work anymore?

I don't understand this I have posted twice on AVSIM and still no way for me to get it working again. I put my flight plan into the FMC and I take off just fine and put LNav on. LNav engages fine most of the time. The problem I am having is when I get to a waypoint the plane passes over it without realizing it has reached the waypoint so it flies past it, realizes it has past the waypoint and starts turning back to get it and starts doing circles while trying to get this waypoint. What can I do to correct this?
Thank You