There are two common causes of semi-auto lockup:
1 - An incomplete trigger pull, resulting in you releasing the trigger (and thus the power) before the mechbox fully cycles.
2 - A shitty battery that doesn't have the current to crank the MB around in a timely manor. 2 can further aggravate 1.
Mechbox lockup is particularly noticeable in V1 and V2 mechboxes, and somewhat less in V3. I haven't had a V6 lock on me yet.
So how can you solve this? First, make sure you always completely pull the trigger, all the way back to the stop, before releasing it in semi. If you find your mechbox takes too long to cycle this way you need to find a way to speed it up. Do this by making sure it's properly greased, shimmed, and that you're running a good battery. The ones that come with JG guns are crap (by way of comparison, the 9.6v 1000 mah JG one that came with my P90 functions only slightly better than my Elite 8.4v 1500 Mah ones. My Intellect 1600 Mah 9.6v blows them all away -Even a trigger tap provides the power to cycle my G36).
You can find a much more in-depth description of this issue here: http://www.airsoftcanada.com/showthread.php?t=25310