I'm trying to distress a font.
I have created outlines on the text, then using brush, drew a line of charcoal brush.
It works but only the first letter of the text is left on the page.
Notice on the tooltip that pathfinder minus says "minus front". It can subtract many things from one thing - out of everything you have in a selection the one thing in the back is what it subtracts everything that's in front of it.
When I say "one thing", groups don't count. When you expanded the text you got a group, and expand text puts the first letter of your text at the very bottom layer.
Then you had all the items in the group selected with your brush work. So what you wound up doing was taking that first letter and subtract everything on top of it. You were confused because since none of the other letters overlapped the bottom-most one, they didn't have any effect in the subtraction. If you try it again but nudge one of the other letters over so it overlaps the very first one, you will see that other letter chew a piece out.
What you would like is if you could combine all those letters into one shape, and then subtract other things out of the combined shape (instead of letters from each other). What is the way for Illustrator to take individual paths and make one compound path out of it?
Object -> Compound Path -> Make
So do that to the expanded text before you do the minus front.
If your brush stroke was coloring all inside the lines you could have used "Exclude" not "Minus Front".
The title of your question is "how to distress a font".
The body of your question is "why doesn't Pathfinder do what I want".
I like the first question better, so I'll answer that.
Play with the Live Trace options and you'll find you can get very different results from the same scan.