There's a clog in your bagger tubing somewhere downstream from the boot on the mower deck. Start at the back of the bagger, put your arm up the outlet and see if you can feel any sticks blocking it. If not, take the hose from the blower to the bagger loose, look inside. If nothing's there, take the whole tubing from the mower deck to the blower off, shake it all out, make sure there's no sticks or anything blocking that. It doesn't take a very large stick to block up the bagger on my B.
Btw, also, the stock blades on kubota decks dont work well with the bagger, you got to get some high lift blades.