SRAM force rear shifts go in the wrong direction when on largest cassette
If your rear derailleur is in the 28t cog, there is only one click available, because shifting to the next (non-existent) cog would put your chain in your wheel. 

Because of the way the Double Tap levers work, the first click will release to the next gear down, either 27t or 26t depending on your cassette.

There is nothing to fix. This is proper behavior for this group set.