Hi Tappy,
It is tough, and pretty stupid. The issue is within the difference between a B1/B2 and the VWS.
In that the VWS gives you very few rights, and allows a stay of max 90 days. The B1/B2 gives rights to stay up to six months (and then apply to extend to a year)... and is the first channel for a lot of people in eventually applying for a green card.
I can see where the embassy are coming from in a way - if your case is anything like mine was. You are not really going their (on a B1/B2) quite on holiday, you are to an extent going to live with your girlfriend. Who you might get engaged to, might marry, might then apply for a green card.
I went to the States at least 6 times in a year on the VWS, for a total of maybe 8 months until they accused me of 'abusing the sheme', 'working here illegally', and telling me I could not use it any more.
You would have thought a mortgage, bank statement and job would be enough to prove 'strong ties' for heavens sake. Did you enclose proof of all of this with your application? An employee letter would definitely help too ('To who it may concern, Tappy is in full time employment with us, we understand he is hoping to visit the US on holiday from x date to y date, and then return to work').
Plus, do you have any family here?
This really should prove enough. If not I think the embassy should be telling you really quite specifically what their issues are.
Good luck.