Yeah, I've noticed the same thing. Wedge will start off critting at around 18k,increasing to around 24k if he has the Crit Damage Up from Biggs. Then suddenly, for no apparent reason, he'll max out at 13 or 14k. Rancor still shows Defense Down debuff both before and after Wedge deals his damage.
I suspect what's happening is the Defense Down is not actually up, despite the icon being present. Wedge's basic says Defense Down is only supposed to last until the end of Wedge's next turn, which means if the Rancor resists the effect on Attack #2, the Defense Down from Attack #1 should end immediately. When Wedge makes Attack #3, there is no Defense Down anymore and thus he does not get the damage bonus. This is ultimately the result that we're seeing but it's confusing because the Defense Down icon never goes away, even if the Rancor is no longer affected by it.