Do you use mods?
If so, turn them all off and try again.
Then turn them on one by one.
There are prerequisites for Fletcher to spawn and then build his base.
Certain mods will violate the prerequisites by, for example, changing the energy or mass storage.
This causes the script to stall.
I had the same problem with the last stage where certain mods will prevent the map from expanding.
These mods didn't cause problems on other campaign maps.