Hey simon.
I have the exact same scope, and mostly the same eyepieces (I have 9, 15, 25 and 32mm GSO plossls), with the same 2x GSO barlow.
First conclusion: It's seeing.
Like you, I can't get my 9mm barlowed to focus properly *most* of the time, that is, when the seeing is any less than
great.
I've been looking at getting a planetary eyepiece around the 7mm focal length, because it'll give a bit more mag than the 9, but not as much as the 9 barlowed. So when the seeing is fairly good, I can use a 7, and when it's GREAT, I can use the 9 barlowed. I also have the option of using my 15 barlowed, which gives me the equivalent of a 7.5mm to use when seeing is above average.
Collimation is definitely something to look at - squaring your focuser etc is all well and good, but what about your secondary and primary mirrors?
What does a startest reveal? Have you gone inside and outside focus in a startest to check for astigmatism?
Have you checked for pinching on your primary and/or secondary mirrors?
A good eyepiece will also make a slight difference but it won't make the seeing better, and it won't make your collimation or astigmatism better. They're the things to check first before spending money on a new eyepiece.
And finally, don't expect to see visually what I (for example) produce with a webcam. Even with the "ok" images I produce, I don't see that sort of detail (or image scale) through the eyepiece. The webcam is just so much better at capturing the moments of good seeing because of the frame rate, and you throw away the blurry ones when you process it. Unfortunately when looking through an eyepiece, you'll only get the tiny-est of glimpses of detail in good seeing - you don't have the benefit of throwing away the memory of all those bad blurry frames and keeping the memory of the good ones
Hope that helps.
PS: Sorry about the novel