Every time you prepare to play the theremin you will be dealing with all kinds of inconsistencies, but your playing position and the theremin's electronics are big hitters. And even ruling out things like hangovers you will also just have off days where nothing clicks, and if you play other instruments you will have experienced that. The difference with the theremin is that keyboards and guitars are much more forgiving of slight positional errors, and in fact every other type of physically-touched continuous-pitch instrument provides at least some references that the theremin does not.
To approach some level of consistency on the theremin you need to be able to work on the your physiological "control loop" that uses muscle memory (this is the open-loop part) to jump to approximate pitch, and then augment that by using your sense of pitch recognition along with minor finger corrections to seamlessly put you precisely on pitch (and this is the closed-loop part). I use the term "precisely" loosely here; this is a theremin after all. If you are familiar with how optical or magnetic drives can precisely position their heads so rapidly you will understand that this is also how the human body should work. In a drive an arm that carries an optical or magnetic head is rapidly moved into an approximate position by open loop control and from that point the optical or magnetic head itself senses the relative track position and makes fine positional adjustments to minimize error. Your arm and the muscle memory in your hand puts you in the approximate position, but from there it's up to your brain to sense the error and rapidly adjust your fingers to null it.
Practice interval jumps relentlessly to train yourself to jump to the vicinity of the correct note rapidly, but understand that because of the variables involved muscle memory can only get you so close. Your pitch recognition and correction loop must be fast to correct the remaining errors, and this involves lots of practice - not just playing practice, but pitch recognition practice (these can be one and the same). I find that it works well to practice interval jumps by following a keyboard recording, or simply play along with other music that has the melody that you want to learn (not suppressed as with karaoke tracks). Things that can sound just fine without a reference can sound pretty bad when played alongside one, but having the reference does force you to sharpen your skills.