I'd check the entire system although the slave cylinder is usually the most likely suspect. Check it just to be sure (even though it was recently replaced).
If the slave checks out then take a look at the clutch return spring (also known as the clutch power spring). They have been known to fail on occasion.
If the slave and return spring are ok, then check the master cylinder. If that is ok, then you may have to drop the transmission.
#9 in the diagram.
