In Sanctuary I have a pump farm and some industrial purifiers going so that it's producing 90 water with 15 inhabintants.

But no matter how long I'm gone for when I come back there's the same 14 units of Purified Water in the workshop inventory. There's no accumulation at all.

At first I thought maybe it's all going to other settlements, but all of my settlements produce more water than they have people, generally by a 2:1 ratio. So I don't think that's the problem.

Your thoughts on what I'm doing wrong will be appreciated.

Not sure, I'll check mine tonight. Pretty sure I way overproduce.

It may only produce enough for each settler that you have. This is purely a guess though.

Did you assign someone to the water pump?

I don't see an option to assign someone to a pump. Unlike with the crops, where food production goes up after assigning someone (NOT after just planting it) water production goes up as soon as I drop the pump in the ground. Also if I highlight a settler and then go to a pump or purifier the command remains "go", not "assign".

There isn't any way to assign someone to a water pump, it self manages.