Over the years (and quite recently) I have heard, read all sorts of fancy explanations why trout sometimes only pluck at flies. Apart from truly missed strikes short takes have one thing in common. They happen mostly on flies that are retrieved i.e. streamers. Over the last few years I have done mostly down stream fishing on the Tongariro and have frequently felt fish plucking at my swinging flies prior to a solid take. Here is my simple explanation for those so-called short takes. Generally flies swing around quite slowly so a fish intercepting them is in no hurry and need not attack them at speed.
Trout like most other fish generally suck in their food items by flaring the gill plates.
Now imagine a trout following a slowly swinging streamer and by flaring its gills expecting to suck it into its mouth. It wont work as you the angler keeps pulling the streamer in the opposite direction and the fish either misses the fly or can not get it right in. Sometimes I have felt two plucks before the weight comes on of a fish that is determined to eat the fly by attacking it. Bumps during the swing with no hook-ups are more likely made by the fly knocking on rocks/sticks rather than by fish missing the fly.
Rainbow