Hi fish4life
I know exactly what you mean.. Sometimes you get to know a water and a few different tactics that often seem to do the trick, but then every now and then, in what seems to be good conditions, the fishing is nothing what it usually is, or the water seems dead.
Like mentioned above, this is fishing and i think you understand this.
I have found that atmospheric pressure plays a big role in the feeding of trout.. High and Low pressure systems determine, to quite a large extent, what depth the fish are holding at, and therefore what they will be feeding on. It is sometimes very useful to get a barometer reading before heading out, so when the usual tactics seem to be failing you might have an idea as to why.
Fazzer suggested doing the opposite of what you might be doing. I think this is a very good idea. On one or two occasions i remember getting so frustrated with not getting any results, especially when you can see fish moving and then nothing seems to work, i would stick on something completely out of the ordinary and then...Bam..!!
Hope this helps
D