I’m wondering if your “Deep Relaxed Mouth Position” is actually a bit too deep? Another helpful word for the technique is the “Semi-Pucker” technique, because it’s mostly the top lip that should be deep on the harmonica, the bottom lip puckers so that the sides of it are blocking either hole adjacent to the one you are playing, if that makes sense?
As long as the harp is touching the moist inner-part of the lip, you’re good. No need push it further than that.
Don’t be discouraged. This is perfectly normal part of the learning process. Every new technique I’ve learned on the harmonica from isolating notes, to bending notes, they all went through periods of “Oh I got it!” followed by “Wait, why the heck can’t I do it now? I thought I had it?”
Totally normal part of the process. Keep the faith. You’re gonna be rocking and rolling before you know it!