I think there are some bugs in this game. It keeps telling me i have an unassigned plasmid slot, wich I don't (and can't access anything at the gene bank). I've also been trying to rescue all the little sisters on the current level, except there aren't any, just a couple of big daddies wandering around alone.
Changing between weapon and plasmid is a royal pain in the backside, frankly. BY the time you've swapped from electro schock to weapon the target has recovered from being zapped.