constantly readjust his embrace, possibly combined with becoming or feeling tense while dancing (which I often interpret as displeasure since if they are at or above my level it's probably not nerves/fear) and if this increases with mistakes I may make, interact as little as possible during the tanda and can't get away fast enough when it is done, then I tend to presume I was probably the recipient of a "charity" dance of some sort and that it is not that they actually wanted to share a dance (or at least became that way), but they felt they had to for whatever reason unbeknown to me.
If the partner is wiggling too much the way you described I sometimes ask: "Are you OK? Is everything fine?" There might be something that is bothering him indeed, and if there is I can also suggest "Do you want to sit down?" . There is nothing wrong with that, he will see that I care, and perhaps we can try again in the future.