Global Adjustments can only be applied to the full project or to individual bills. If you want to make adjustments to specific items within the bill, you will need to use Item Adjustments. The easiest way to do this is as follows:
Note: If, for whatever reason, you cannot do step 2, or you only have a few items, you could also use CTRL-click to highlight each of the items in turn, but if you have a lot of items, this may mean you would have to work in sections to avoid accidentally deselecting everything you've just selected. If doing it this way, skip steps 3 and 4.
Check which flags are currently in use in the job, if any.
Apply an otherwise-unused flag to mark the items you wish to change
Use the funnel icon at the top right to filter the project to just the items that are flagged.
Click Select All from the Range menu to highlight all the visible items.
Select Adjustments from the Range menu.
Enter your chosen percentage or cash adjustment to the relevant resource type(s).
Tip: It is advisable, before making any such change, to make a backup of your project.