That happened to me last month.

Probably leaking fluid from the slave. At the very least, you'll need to replace whatever is leaking -- master, slave, or hose, and bleed the system (I had a mechanic do it, but it was claimed here that it involved black magic. Sacrifices of goats with stone knives, incantations, and all).

The fact that my mechanic had 2-3 tries before he got the clutch 100% right makes me think that those opinions are correct