A couple things.
First, is that subroutine actually being scanned? There needs to be a JSR (Jump to Subroutine) that references that ladder if it isn't the Main ladder.
If it is the main ladder, is it set to be the main ladder? The icon in the tree will be different than the other ladders in that program. Non-main ladders have a box around their icons while the main ladder doesn't. If you need to set it up. Right click on the program in the tree (the level above the ladder files) and click "properties". One of the tabs in the dialog box will have two drop-down menus allowing you to designate a Main routine and a fault routine. These are not always populated by default and have to be done so manually.
Second, are there any other destructive instructions writing to that same tag? When you right click on the tag and click "Cross Reference," the window will pop up showing all the instances of that tag in the program. There is a column called "Destructive" and each instance will either say yes or no. If there is more than one entry that says "Yes," this is the likely culprit.
