I can explain this one!
The game is actually really, really finicky about where you place them, dropping them on the wooden posts (here) :
View attachment 3871977
works best.
also, contrary to the pic above, don't use Nuns or shortstacks for defence, they are well... really bad at their job.
Shortstacks seem best if you want to pass goblin stats to a human female (via MC)
Nun's are great because they can produce archer offspring (with the MC) that only have 1 deployment cost.
*edit, nvm, seems you do need to drop on top of an existing unit for them to be usable.... lame