Possible Repeat Carbonation Issue

Back in February I brewed up a batch of Ed's Bee Cave Hefeweizen (somewhat modified through Brewsmith for partial mash, partial boil, late extract addition). There were no issues on brew day, hit my temps, etc. I left the batch in the primary for three weeks, then bottled. After three weeks in the bottles, at 70-75 degrees, there was absolutely NO carbonation. I did not get too stressed, just let it sit for another three weeks, plenty of other stuff in the pipeline. But, still no carb. We went on vacation, let another three or four weeks go by, still nothing. I replayed the mental tape of bottling day, and I definitely did not forget the priming sugar. At this point I opened up all of the bottles and added just a SMALL bit of Notty to each, based on my belief that the priming sugar was in there. Let another month pass, voila, that's some dang good beer, nicely carbed. Yay!

But. Back in June I brewed up a batch of Ed's Haus Pale Ale. It kinda got covered up with other batches after bottling. ( I use tall plastic containers intended for hanging file folders. They hold 15 bottles very nicely, and have lids that permit easy stacking. And, if a bottle happens to break, which has never happened to me, no mess.) Anyway, cracked one open a couple of nights ago, flat, flat, flat.

And again, there is no doubt that the sugar was added. Any ideas? I thought that maybe they were in the primary too long, but come on, it's just three weeks! Kinda stumped at this point. BTW, I'm not an oldtimer at this, but my handy-dandy log tells me that I have done over two dozen batches a this point. These are the only two that have remotely behaved this way.

Another BTW. The Hefe was made with the Wyeast hefe yeast, and the Pale Ale with Notty.

I personally think 3+ weeks is too long and too much yeast is dropping out of suspension and leaving you little to perform secondary fermentation. I use cornie kegs and force carbonate so I don't suffer this issue but one beer that I've left in the primary FV has been there for 17 days and early tastings suggest that the dead yeast cells have killed it, it has a very off taste. Time will tell on that.

If you're bottling I wouldn't go beyond 14 days and you could easily bottle after 10 days and still get clear beer. Leaving it as long as you have is likely your problem as there's virtually no yeast left in suspension.

I have left beers in fermenters for a lot longer than 3 weeks and never had any problems carbing in bottles with priming sugar. In most cases, I ferment in primary for two weeks and then secondary for two week on all of my beers, and I run a longer secondary on some heavier beers.

I would verify that the thing in your bag marked "corn sugar" is truly corn sugar. Another poster recently told a similar story to yours: what he thought was corn sugar was a wine mineral that looked exactly the same.

I just bought some lactose yesterday and it looks EXACTLY like my corn sugar. I would check just to make sure..

You definitely aren't leaving it in the primary too long. I have left beers in primary for 8 weeks that have carbed fine and haven't had any issues. I leave every beer for 3-4 weeks.