If you've got multiple "stay back if needed" set as the instructions, it is likely that some or even most of them end up using the fallback instruction (go forward or lurk outside area) because most of the opposing teams nowadays only leave one player to stay forward when defending a set piece. In some cases they might not leave anyone up. If you want to make sure both of your fullbacks stay back for a set piece, set two instructions in the Recovery Defenders to "stay back" (not "stay back if needed") and make sure your fullbacks are top of the priority list for Recovery Defenders. And then you can bump the Recovery Defenders up the category priority list (on the right side of that pop-up) if those fullbacks are still getting assigned to other categories.