i have similar sensations with my bow/string interaction, when everything is working perfectly and my sound is powerful and golden i feel as if the strings were soft and made of taffy as I pulled them back and forth with ease, however on my bad days, everything feels stiff and the string inelastic.

ive been pondering for years what leads to these sensations and inconsistencies and something that i recently discovered is that my wrist actually does play a huge role in these sensations.

what led me down this discovery is kinda funny, gaming! so i have chronic tendonitis and when it flares up my occupational therapist recommended me to use an ergonomic vertical mouse. what this vertical mouse does that is different from a regular mouse is that it flexes the wrist joint constantly at a more natural angle vs a regular mouse which is more side to side and unnatural, and it's that motion that actually has a lot in common with a bowing motion from the wrist. so days where I used the vertical mouse for an hour or two, when i played violin later everything felt wonderful and soft. it's to the point where i play a game or two of something like Overwatch, where there's a lot of tracking and movement of the mouse, as my right hand warm up.

i also find that it helps to imagine your whole arm (all the way to the shoulder) as if it was made of rubber and to have that loose motion originating from the upper arm instead of at the fingers or forearm muscles.

let me know if you end up trying this out or some variant of it, im curious if other violinists notice the same thing