Honestly I've never heard of a problem with that method, i've done a couple siphons with a short tube where I held the bucket up and then lowered it when the liquid level was higher, so that after the halfway point the tube was in the middle of the bucket. I doubt any of your beer will be flat, the beer might have just tasted sweeter than you thought because it was flat at the time or needed some aging. Then again I'd think you'd have some bubbles at 10 days, are you storing these at room temp? Are you sure your yeast is viable? I see you have an Alt in your sig , doesn't that mean a long cold conditioning? Maybe you need some fresh yeast.
I've forgotten the priming sugar, remembered halfway, dumped it in and didn't stir and I still didn't get any flat beers, unless I just got lucky...
Primary 2: Air
Bottled & carbing: Nothin
drinking: Poet on Payday Wee Heavy
gone but not forgotten: Wilford (Oatmeal Stout) Samhain Surprise Porter, J's Mirac-Ale (Brown Ale), Altered Beast Ale (German Alt), Chucks Not Ballys Cali Common, S'Wheat (Honey Weizen), NIHs (Numerous Identical Hefes), Wittler Farms Maple Brown, Failed Experimental Yarrow Beer, O'Mega Red (Irish Red)