You could of just been experiencing 'one of those days'. It could of been a series of factors, barometric pressure, muddy water, cooler water temp ect. Some things to consider trying when your faced with a post front or cold muddy water is still fishing with cutbait, chicken livers and even sqiud.
When your working artificials work them slooow, and have you ever used chartreuse on this lake?
We had some slow days fishing for hybrids last week and with conditions just like the one's you were faced with, and as you already know, we just need for the water to warm up a bit to get them turned on!
I then snagged a huge shad, close to 8". I've never seen a shad that big.
Had black dot behing eyes/gills
It sounds like a hog threadfin shad seeing how you mentioned the black dot on the gill covers. Threadfins usually average around 6" and are more streamlined than gizzard shad. Gizzard shads are more plump in appearance and I have seen some gizzard shads 15" long.
As far as hybrids feeding on 8" shad, I'm sure there are some hybrids in the river that won't have a problem munching on them. I personally favor using shad in the 4-6" when targeting hybrids. Shad will start spawing when the water temps start reaching 60 degree's, and thats when you will really start seeing them up the creeks!
BTW..You ever fish Brookville reservoir over here in Indiana?