Rear mech not shifting into biggest cog

I've just installed a brand new force 1 11speed rear mech on my bike but I cant get it to shift all the way up to the lowest gear. I can get all the way up to the second to last sprocket but then when I click the shifter again it seems to move over as if to shift up but then immediately springs back.
If I push the derailleur manually it will move all the way out and I've set the limit screws so that the jockey wheels are aligned properly. I've tried adjusting the cable tension but that doesn't fix the problem either.
The shifters are sram force 11 speed, the mech is sram force 11 speed and its a sram 11 speed cassette so its not a compatibility issue, anybody got any ideas as to where I'm going wrong?

When you shift into the largest cog and the derailleur springs back, is the shifter still in the lowest gear position?

Start again. Loosen off your limit screws, undo the cable and shift all the way down to slacken the cable, then reattach with enough slack taken up slightly. Then set your limits. Sounds like you started with the shifter not fully backed off, so its indexed one cog over from where it should be

You're missing one shift, add tension.

Thanks for the help guys. I've got it going all the way up the cassette now by adding more tension but now its not shifting back down. Is it just a case of fine tuning the barrel adjuster until it works both ways?

If the chain, cassette, cable and housing are in good shape, then yes.

What I have found helps is once you have attached the cable and before you set the limit screws and indexing is to pull the cable by hand to make the shifter changes gear, keep pulling until it reaches the biggest sprocket, then release the cable to let the spring take it back to the smallest - then you know the mech's range for your cassette is fine. Then set your limit screws and check again by pulling cable by hand to make sure it goes from smallest to largest sprocket and back and no further. Then start on the indexing (using cable tension).