The higher octave definitely requires more finess in regards to technique.
Like Ron said, make sure you are relaxed. The higher notes require a fairly exact mouth cavity shape to sound properly I find. For a long time I would squeel the higher notes (only on a particular brand of harp), and I thought it was due to the reeds twisting or rubbing in the slot. Turns out it was just me! My tongue was too far forward for the draw notes which did something to ruin the note. Fine on the blow, just not the draw. So after a bit of shuffling around I figured it out, and if I dropped my tongue they sounded strong.
So my advice, keep practicing and play around with embouchure, tongue positioning, force, etc.
Finally…. If all else fails there is a small chance that the reeds are too closely gapped, meaning that the reeds inside the harmonica are closed almost shut, with no space at the tips. This can be great for good responsive playing but can cause the reeds to not sound if it’s too tight. Mitch has a video on this I think in the tool shed? It’s not hard to fix if your harmonica is kept together with screws.
Anyway, good luck!