What I meant to say in my post above is that I did grade 8 about 2.5 years ago (I deleted the 8 by accident)
So, what pieces are you trying to sight-read ? Could you learn them easily (note wise mostly) if you wanted ?
For examination purposes (if I ever get there), I'm supposed to be able to sight read music of about grade 6 standard in any style. This is just a nightmare - it could take me about a month to play some of these pieces correctly, depending on the style, but will only be given 1 minute's preparation at the exam. You're right that I probably have to "specialise" a bit more in a particular style at a time rather than try everything in one go. This is a good idea I'm going to take up. For example, I've never played much Bach so find it almost impossible to sight read anything with counterpoint. I probably ought to learn some music in this style in the first place.
I don't find Mozart and Clementi so terribly difficult to sight-read by comparison, but this is because I've already learned several sonatas in this style. Apart from some very difficult Mozart, this type of music is oftern linear, with certain patterns that I can recognise.
Liszt is a composer I can't play at all (I'm trying to learn the lovely Consolation No. 2 but am getting nowhere with it and have started to despair despite the title!). However, I find Chopin more intuitive in general.
Recognising chords is a particular problem. I am probably being really dense about the whole thing because I'm assured chord recognition is really simple, but despite the best advice in the world, I have this terrible habit of perceiving chords as independent superimposed notes (or a mass of black dots at first!), so have to think individually about what each note is, and this takes a long time. My teacher recommended that I learn to play some cadential sequences in different keys, and I've been doing this for weeks, but it's been mostly pointless because I've never come across these specific combinations in real music.
I shall be looking for some intermediate Bach to start this evening and see how that goes...