if your tranny otyherwise shifts fine other then 1st going to 2nd then your clutch maynot be the issue..
like cteng said,,sounds like its inside the tranny if it shift in other gears ok..
seems to me if it was the clutch,slave,or master cyl then your problems would be through out your shift pattern..not just two gears..if your having this problem through all the gears,,then yea,,maybe the pressure is bleeding down,,weak or has air..