As others have mentioned, the barrel has to be totally grease/wax free for the Plum Brown to work without making streaks.
I've taken to wearing the heavy kind of rubber gloves sold for washing floors etc at the local hardware stores. These keep the oils from my skin from contaminating the barrel.
I used to use steel wool between coats to "card" down the surface but I found that steel wool does two things. First, it will contaminate the surface with the oil/wax it is coated with to keep it from rusting. Second, it often removes more of the browning than I want to remove.
To get around this, I wash the residue off of the barrel between coats using wet paper towels.
These are oil free and are abrasive enough to remove the deposits left by the PB.
As others said, count on applying at least 3 coats. Four are better.
All of this washing and reheating contribute to the amount of time it takes to brown the barrel with PB.
After 3 or 4 coats, the barrel will still look rather uneven but it does get better.
I would suggest that you let the barrel "rest" overnight after the final washing. The next morning (or so) wipe off any rust that has formed with a wet paper towel, dry it off and then apply a coat of oil like Sheath.
The browning will get noticably darker and will look more uniform.
As others have said, using Laurel Mountain browning gives a better browning but it does take several days to do the job.
Have Fun!
zonie