I've experienced this phenomenon in SW games on two occasions, one as the GM and once as the player. Poor luck and poor player synergy can lead to the problem, but after I encountered it as a GM I shied away from the system for a while (we're talking all the way back to original 1E SW days). I eventually came back to it, and settled on some simple rules of thumb to keep things moving. The first one was: keep my players informed; I know the rules pretty well, and I remind them (or provide useful cards for reference) on their many options. If they aren't taking advantage of a certain course of action, I point it out....sometimes the players are just not great at thinking of these things, especially if they're used to more "static" games like D&D where tactical synergies aren't as common or necessary. The second rule of thumb is far more important: if I notice a toughness value or some other stat I have control of is getting in the way, I drop it a point or two. I don't share this stuff with the players, I just recognize that I have overbalanced the game, or am using a stat block that is weighted against the group as they are functioning, and I do an on-the-fly fix. I know some players might not like that, but usually when the group is having this sort of trouble, it is because "that type of player" is not in the group in the first place (if they were, they would be advising their cohorts of better tactics, usually!)
As a side note to the OP: you mention they were at best getting shaken conditions on the extras. Were they then failing to get a second shaken condition? Two shaken conditions in a row turn into a wound, eliminating the extra.
Ultimately though I usually have the opposite problem in SW (fantasy, at least), in which the players manage to wipe out the opposition with overwhelming firepower and coordination. It's a sight to behold when they clear a room of two dozen foes in three rounds and 20 minutes, and realize that would have been a 4 hour session in 5E!
