Before trying to cap with one pass you need to check the WPS to see what the maximum bead width allowed is. Most of the time its not a matter of if you can or cannot carry that much metal because the WPS doesnt allow it.
Most WPS's have a 5/8 max bead width for SMAW.
I agree with above posts that your travel speed on the root pass is to slow, and you need to have the least amount of starts and stops as possible. As for the cap, it seems like your having trouble keeping your beads on a straight line, you could always grind a line to follow.
Just keep practicing and you will get it.
Yep! I agree. And I use the "grinding a line" technique always.