Hi Adam,
I am also a relitive newbie to this dark art but I'll pop in my 2c for what it's worth. I have a 4" scope with a similar focal length. When I first started viewing the planets, I experienced a similar problem. I was looking through a 6mm neagler lense lent to me by a friend. The moon was incredible with great crater detail and beautiful shadows cast at the terminator, alas, any other solar object was an undefined ball and were extremely temprimental to focus. I ended up getting a set of eyepieces that included a 5mm 13mm and 21mm. I used the 13mm and although the planets were smaller, they focused well and gave me 10 times the detail. I packaged that with a 2x Barlow and hey presto, bigger planets and same detail. The 5mm is amazing on the moon but impossible to focus on planets. I guess what I'm getting at or that I figured out at the end of this experimenting is that once the eyepiece gets close to your scopes limit the image falls off the proverbial cliff
Only extra that I would add is that there is also a huge difference in seeing ability once the planet gets about 45 degrees above the horizon due the the massively reduced amount of atmosphere you're looking through before getting to the black stuff !