Other people (including dealers who sell them) find the Zen-On bass unresponsive high up, so it isn't just me. I know enough to not just try the standard fingerings but also use alternatives or make up my own. This is a common problem with wide-bore basses - the Zen-On is more like a Renaissance recorder than a Baroque one. I once tried a Mollenhauer Kynseker bass and it was even more extreme - fantastically loud in the low register but nothing would make it play above C in the upper octave. Doesn't matter for Renaissance bass parts.

Recorders are idiosyncratic things. The hard note at the top end for the Zen-On is D (t12- ----, like the upper A on a C recorder). It sounds more easily both above and below that. This is a fairly common problem; the Aulos plastic tenor has it too.

I expect the problem will lessen with time as the right combination of breath pressure and thumb opening becomes instinctive.