Phil, this also can happen if FS reloads all the .bgls, ex: you are switching scenery. I found a fix (in FSX) by just taking all traffic sliders to 0%, return to the sim, then go back in to the options and place your traffic slider where you originally had it.
Many thanks to you all, it was very kind of you to help me, I did mention to Greg who was helping me with a problem, that I am a retired aircraft engineer I know every bolt on most aircraft, but computers and me don't get on.
I will try putting the traffic slider to zero, do I have to close flightsim down after.