I have play fallout 4 for sometime. I just finish Act.1 On the main quest and hold it for build some settlements. I try to find some Level Four Traders for the settlements. I keep travel to place that have Random Encounter spawn. But when I keep travel to those place 5-6 times later I stop face the encounter, the place is EMPTY. I have try to wait 10 game days to make the cell reset and go back to those place and see it still EMPTY not have random encounter anymore.
Can someone tell me what happen, and someone know how to fix this ??

Thanks and sorry about the language.

This is a tricky one.
I know of one or two spawn points which are good for meeting NPC - (Carpark behind Concord, and the road down behind Cambridge police station near the fallen billboard, as you approach the corner of the building passing the billboard on your left.)
The tricky thing is this - sometimes, on certain characters, they work every time. And sometimes, on a different character, different playthrough, they don't work at all.
I guess it is a script fail that occurs early in the game, preventing NPC from randomly spawning ever.
I know one of my early playthroughs I never saw ANYONE at the carpark behind Concord, and then my last two characters I have met someone there (including mobs), each time I passed the location.
Like I said, it is tricky.

Look like it's happen in place that I visit many times. The place that you mention road behind Cambridge police station is EMPTY right now another place is road from Lake Quannapowitt water treatment plant is empty too.