I have an issue with the 7 speed Shimano Altus/Shimano twistgrip set up on one of my bikes.

By altering the cable tension with the barrel adjuster on the rear derailleur I can get 6 of the gears, but never all 7.

I can set it so I get the smallest cog but not the largest or vice versa.

Finding it quite difficult to figure out!

Relevant info:
It's the gearing that came with the bike from the factory
I've put a brand new gear cable in
I'm assuming the limit screws are correct as it can shift to the relevant cogs ok as long as cable tension suits. It never shifts off the cassette
The hanger is a pretty solid piece of steel and doesn't look bent

Any ideas?

Worth getting hold of a mech hanger alignment tool. The hanger might look straight but could be subtly out. It's a bit like trying to build a house without ensuring that the foundations aren't dodgy.

Are you missing a click off the twist grip? Does sound like it is a shifter issue rather than at the other end.

Count the clicks have you got them all ?
Check rear mech adjustment is correct ( if you can get top and botto hand shifting tben shifter must be the issue)
Check cable run is smooth and not kinked somewhere
If those are correct and have 6 clicks so 7 gears it should be usable.
Bent hanger is much more likely to skip gears/ change poorly than drop either top or bottom.

Cable the wrong side of the clamp bolt in the rear mech, effectively reducing the cable pull per click...

Went back to check a few things:

The cable seems routed correctly from the handlebars back - can't see any kinks or odd bends - it is still running through the factory outer
The shifter has 6 clicks
The cable is routed through the clamp correctly
With the cable unclamped from the derailleur then the action at the twist grip is very light/easy, but once the cable is clamped it is much stiffer/harder to change to larger cogs.

After fiddling around with the cable tension I can get it going through all the gears but it can take quite a few revolutions of the cranks to drop into the smallest cog or settle properly on the largest one and mid-cassette shifting sometimes doesn't work or needs you to add a little tension past the index point to get it to jump a cog...
If you want "proper" shifting (i.e. settles into each gear properly just after you click) then you lose either the top or bottom gear.

Sounds like either you need new cable outers or the derailleur pivots/spring need cleaning.

Check the ends of the outer cables, under the ferrules. I had a stupid issue with shifting on my road bike and couldn't find it for ages, turned out the outer had split right where it left the shifter so it couldn't be seen... but compressed lots when used and gave a similar issue to what you're finding.

I have got it to work!

After pulling off the ferrule at the mech end and disconnecting the cable, I could feel that the cable was binding a little when tugging on it, felt like a snag or rust or something just inside that end of the outer.
Some liberal application of silicone spray and tugging to/fro has sorted it out and it is shifting nicely again!

Glad I don't have to change the outer as it goes inside the frame for no good reason...

Thank you everyone, I am a happy chappy now :)

Sounds like the outer needs changing..

For internal routing there’s a tool you use for joining the old & new outers & pulling the new one through. It’s something like this-
https://www.ebay.co.uk/itm/285784654657 ... shQg4_jnTp

What's Wotsits said above.