Unfortunately, there are many places for wear in the Series clutch mechanism. 1st thing to do is to have someone depress the clutch slowly (then rapidly several times) while you watch from underneath. Make sure nothing is visibly loose, nuts, bolts, slave cylinder itself, etc.. If nothing looks amiss, here are some other things to check.

How far down does the pedal actually move before movement is detected in the slave cylinder rod? If it's more than 2", you probably still have some air in the lines, or your MC or SC are going bad.

The pins and linkage can definitely wear, try to check these as well when watching from underneath.

Another often overlooked point of wear in the S2A is the clutch fork. The contact points of the fork ride on the throwout bearing holder whenever the clutch is held in. If you are prone to holding the clutch in for extended periods (at red lights, resting your foot on the clutch, etc.) or if the part is original, the points can wear down enough that you don't get full engagement.

Lastly, you might try to lightly rotate the linkage with a pair of pliers just to further see if you can spot a worn item.