The only thing I've come across apart from obvious failures is the main pivot bolt being done up too tight... it can rotate on each reset and sometimes there's a tight point which can then find the target refusing to drop. Resetting up and down an few times can rotate the bolt and it's free, only to bind again when the bolt rotates to the tight spot.
Apart from that, snagged strings catching on the ground or foliage, are the only times i've seen a target 'fail' when positioned correctly.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep