Chambers with a pronounced narrowing may exhibit a "pitch jump" effect on the higher notes in the octave. This is done deliberately by some makers for this effect.
Figure A shows a stem-like channel that divides the chamber.
Figure B shows a circular wall that has a small center opening.This also may have a pitch jump if this opening is too small.

Figural ocarina (sitting cat)
Figure C Sculptural ocarinas have odd-shaped chambers which might also have the pitch jump effect.
Figure D To eliminate this, widen the connecting channel (blue arrows)

The whistle assembly or voicing needs space or depth beneath it to work properly.
Figure E This ocarina's chamber is too shallow. The higher notes may sound squeaky.
Figure F This ocarina's chamber has plenty of room below the voicing.