I'm usually really good at finding a problem and fixing it especially with the drivetrain, but this one has me completely dumbfounded. Never had this happen to me before. The problem is: I can't get 8th gear when upshifting or downshifting. This is on my dirt jump bike which is a 1x9. I get 9,7,6,5,4,3,2,1 but no 8th. The cable is adjusted perfectly, chain is the right tension, new as well, cassette is new. Deraillieur is a 9 speed Shimano XT non rapid rise. Limiting screws are where they should be set for the bike, can't see that as a problem though since it doesn't go off the 9th or off the 1st gear. Deraillieur hanger is straight. Anyone have any idea what is up with it at all?

Hmm, what kind of shifter do you have? Try pullling the shifter a little bit past where you'd want it to be, sometimes, this is enough to "catch" that sprocket.

Didn't catch when I tried it earlier today. Shimano XT trigger.

This is going to sound contrary to just plain wrong, but try it anyway, it may work and at this stage it might be worth trying. Make the rear der.cable looser than it should be when there is no tension on it.
Set the limit screw so it won't fall off of 9th. Try downshifting and use the barrel adjusters to adjust the tension. Sounds like it is pulling too much cable going from 9 to 8, so might be starting out with just a little too much cable tension? Once you do get it working, let us know how and what worked if you would.

How long ago was your last crash?

Three days ago, but went down on the non drive side.

Oil the shifter.

the same thing happens to me, except its gear 6. However this gear looks much more worn than the others and i expect this is why. As for now, I just push up on the shifter a little bit more and it catches.

If it was only that simple, doesn't shift hard at all.

Oil the shifter.

Seriously. Spray some light oil into there, and it will help free up all the little ratchets and stuff inside the shifter, which may help your problem. Even if it shifts 'fine' still spray a little oil into there to free things up a bit. What do you have to lose?

Being that I oiled the shifter Sunday when I put the new cable on it, don't think that's the problem. I keep the parts on my bikes in perfect running order, this is starting to drive me nuts now changed everything else but the deraillieur itself, matter as well try that approach now.

How is old is the cassette?

New cassette

So when you shift to gear 8, is the chain trying to hook onto the gear but not making it so you hear that grinding sound? Does the chain simply not even move toward gear 8 when your on 9 and shifting up

Just found the problem.

Care to enlighten the rest of us?

Bird crapped on your 8th cog?

He tightened the nut behind the handlebars.

Have to get a new deraillieur.

Haha.

You would have gotten stumped with this one as well one of the parrallel links isn't exactly parrallel anymore.

That would certainly do it. It's funny how it only screwed up the one gear position