when building station you have to select them differently then when building a facility. Also I think (once again I think) you have to be assigned to the NPCs you are wanting to use, even if you one them.
I owned all of them, had all of them assigned, and they were all waiting in the entrance room of my Corona.
When starting construction on the first, it gave me the option of 3 builders total. I used one, confused as to why this was the case. When starting another, it gave me a choice between the two left from the original 3.
I believe this just happened to me, I was assigned Manager of the builders, but the supervisor was set to none.
I have 60 with me and I am assigned manager of them all, but the 7 that show up on the list I am assigned manager and supervisor. Since I was manager I just assigned myself supervisor and now they all show up for me to choose from.
I just built 3 shipyards the other day with 25 builders each. Black Curs owns them, Slayn and Korpil were assign manager and I was supervisor. Aside from that they were all in the entrance room of the tabder I used to build.