I think the issue is there are forces at work we are yet to understand about fish!! A lot of my fishing is carried out in gin clear water which gives a uniques insight into what is happening beneath the surface.
For example, when trotting maggots, the most eager fish from the shoal are caught - often the smaller ones. you may catch a few and then the swim goes dead. you could be mistaken into thinking the fish have diassapeared and moved downstream - i know i have! In visible water the scenario becomes clearer. The fish are still feeding, however, they check out the bait, realise something is fishy and then ignore it and start eating the maggots on the bottom of the river bed - there a safer bet. I'm assuming your presenting the bait well (I try!)
You can fine down a bit, and again a few bites will rewarded, then nothing. switch to feeding on the bottom, and again the presentation blows. I think what makes match anglers so skilled is that they almost read the water and have a mental picture of what is goin on so they adapt their presentation as a session progresses. Thats a lot of agro if you want to catch big fish - so chuck out a big bait and wait!!!
Back to the forces we don't understand. I am sure fish have an as yet not fully understood way of communicating danger to shoal members (and I 'aint talking telepathic mumbo jumbo here!). Its the same when using heavily flavoured baits! You may catch a few fish on that 'special bait' and think your onto something - but over the season, it becomes clear that the other fish don't want to embarass themselves! How can they know to avoid it? Especially when they havent had the pleasure of a bankside visit? Do other members of the 'fish comunity' behave in a way that illicits an 'avoid that sh*t, it 'ant worth bothering with' responce?
Is this why less flavoured baits like maggots, bread, unflavoured luncheon meat and cheese paste (I will also add halibut pellets in smaller sizes - less than 6mm for the same reasons as follows) continue to trip up the biggies? I wouldn't mind betting tonnes of the stuff is eaten by fish every day over a season. mostly as free offerings and therefore a degree of safety - though I'm sure they would prefer to exploit a safer more natural alternative if nature provides.
In my experiences, big fish can be rediculously easy - or conversely incredibally hard. It is all down to angling pressure and learnt experiences from their own or other fishes experiences I believe.