Here's my understanding of how resources work in settlements linked by a supply line.

Settlement A: 15 settlers, 9 water

Settlement B: 15 settlers, 15 water

Settlement C: 15 settlers, 30 water

When I go to withdraw water from Settlement C it should have 9 water, because its own inhabitants are using 15 and six are going into the supply chain to make up the shortfall in Settlement A. So 30 -15 - 6 = 9 surplus.

Assuming I'm correct here's my question: none of my settlements have a water deficit. The lowest one produces three more water than it has settlers, so if the system works as advertised there's no reason that any purified water should be leaving the place where it's manufactured. And yet when I go to some of my settlements they either have no surplus or a very small one. In fact the only appreciable surplus I see is from my big hitters where I've got 9 industrial purifiers each.

Does anyone know why this is?

I know that there seems to be a glitch/limitation where items produced by a settlement don't "stack" appropriately in the workshop. So, for instance, if you look at the workshop inventory but don't remove the water that is there, no more will be added. However, remove each time, and it should continue to accumulate appropriately.

I haven't played around with it myself yet, but as long as I'm removing what is there each time, I haven't had any issue. It seems to generate in a reasonable amount.

Yup, The boat house has 10 large purified water filters (400 water) and generates 287 purified water a day - if I remember to remove the previous day's water from the workshop.

Ah, ok, so I have to keep them empty. Thanks!