the only thing i can think of you may have been doing wrong was targetting the wrong type of water. If your after springs your going to want the heads of runs where they tent to be deeper and faster where the coho will sit in the tailouts or edges of the runs that. the coho will mix right in with the pinks however
but they do also try to avoid them (depends on the run your fishing)
If you fish runs like the Train Bridge run for example, i have found this usually. This run consists of great water for all species but they will sit in different parts of. this run has a long deep run with a shallower edge on 1 side. this run may look to be the same speed water everywhere but its really not. Here is how you can break it down.
the shallow edge will be the side your standing on (if you drive in of course) now this shallower water will be filled with pinks doing their spawning business, yes some coho & jack springs will be in there but not many. in the deeper part of the run which is the opposite bank (where the big rocks are) its considerably deeper and infact a little faster water, this is the water where the springs will be for sure and yes coho will hug right against the big rocks to avoid the springs as they travel up river. Now the tailout of this same run will hold coho, jack springs, even some of the smaller adult springs and unfortunately some pinks if they can hold in the current. these are not concrete rules to fishing but this gives you examples of water types to watch for and how to break down each run you are plannning to fish.
another great chunk of water for coho to hide in is behind big rocks in heavy chop water. these fish can be very tricky to fish for because of the very tiny drift you get and the fact the water alot of the time is quite deep. you will find this water in the upper parts of the mid river all the way to the top.
Im sure other people will chime in with their own observations and i may have forgot things LOL as i mainly just go fishing to shake hands with some fish and dont specifically target exact types of water unless it for steelies
have fun out there