I would be suspicious of the condition of the connector pin surfaces.

Perhaps, because of age the parts need to be re-conditioned. You do not mention the type of solderability test, i.e.. dip-and-look vs. wetting balance.

We use a wetting balance (MUST III, GEN3 Systems) in our process development lab and often find that wettability curves will vary with different solders, fluxes, temps and time profiles.

IPC/EIA J-STD-003A and other solderability tests call out a specific profile, solder and flux - in this manner the part being tested is the variable. By using variable solders and fluxes, without establishing a base line for the wettability of the part, there are too many variables for a sound decision.

First determine the quality of the parts, then determine the effects of the different solders and fluxes.

It is possible that you damaged the plating by operating your soldering stations at too high a soldering temperature.

I would suggest evaluating a direct power soldering system operating at a lower tip temperature.