Could be that the hydroxide isn't strong enough, could be your application, or it could be luck of the draw.
Hydroxide is best downstreamed, not directly sprayed on. Factor in that dilution ratio when you're mixing it. Give it a solid dwell time. You know it's ready when you can gently rub off the old sealer. Keep it wet for up to 15 minutes as necessary, and if it's still not ready, apply a stronger mix. For fences, or difficult verticals, add a non-foaming thickening agent to your hydroxide. I keep a tub of hydroxide powder in the trailer. If my stripper isn't strong enough, I add a handful or 2.
Oxalic application is pretty simple. It doesn't take much to neutralize and brighten, just be sure to DS it on as uniformly as possible, and rinse the house (especially windows and metals) very well. Give it 5 mins and take a look around to make sure you didn't miss anywhere, then rinse like crazy. When you're done rinsing, rinse again.
But it could just be you're landing some unlucky gigs. Even a very strong SH mix with the consistency of Dawn has a tough time with some solid sealers. Behr, Sikkens, and Cabot come to mind as some of the most difficult.
It's hard to determine what you are possibly doing wrong without knowing the specifics of the problems you are encountering. I see this post is from last October. If you are still around, post some pics or describe what isn't working so we can help or learn more.