Based on a Reddit post that anolyzed many of the formulas in Fallout 4, unassigned settlers and setters assigned to the scavenging station will only add junk to the workbench if the amount of junk in the workbench is below a certain threshold. The formula is as follows:
100 + 5 * settler population
That means if the settlement population is 15, once the amount of junk in the workbench equals 175 unassigned settlers (1 junk) and your scavengers (2 junk) will stop added junk to the workbench. The way around this is to move all the junk from the workbench to another container, but to me that is really annoying. If you want to have access to that junk in build mode, then you will need to move that crap back to the workbench. Then when you are done building you need to move 'em all back to the container so that your settlers can gather more junk.
If the junk is not in the workbench, then that also means if you have supply lines set up between all your settlements, that junk will not be accessible. I simply do not bother building any scavenger stations because I know that in each of my settlements there are at least 500 pieces of junk in the workbench.