Have you performed an inspection of the inlet header? Were you able to spray helium at the top of the tube bundles, right next to the inlet header?
We have experienced leaks at our tube inlets. They are sometimes hard to find with helium leak testing if you cannot spray all the way at the top, or if it is windy, etc.
Also, what chemistry program are you running, and what in what steam pH range do you operate?
I ask this because we found that running too low of steam pH leads to excessive FAC in the STG exhaust header and the ACC inlet header. This was causing a lot of leaks and we had trouble maintaining vacuum for a long time.