When I first got my band, I initially lost 20 lbs as well, but then it started slowing down. I too found ways to circumvent the band so to speak, mostly by drinking and forcing food through. I was getting frustrated as well (I never gained, but I lost REALLLLLY slowly) so I kept going back for more fills every month. I finally hit my sweet spot last month, and now the pounds are melting off. I can no longer force food through, and I can eat very little. My point is, hang in there, keep going for fills, and eventually you too will find your sweet spot. The band is almost foolproof, if you have enough fill in there, you WILL lose.
EDIT: Just noticed I'm using my wife's account who hasn't done it. I did the surgery about 9 months ago just as an FYI.