I go to several different sessions and SingArounds/TuneArounds(!) and really enjoy both, usually. I like SingArounds because I'm quite shy of jumping in in a session and at least I get to have a turn in a singaround. Yes, I know you experienced and confident musicians sometimes can't understand someone being reluctant to jump in but some people are just naturally more timid - it doesn't mean they can't play or that they can't make a valuable contribution!

One session I go to is being wrecked by one person who takes turn after turn - sometimes singing 3 or 4 things in a row. He then lets someone else have a turn but jumps in again straight after. No-one wants to upset him by telling him to shut up and let so-and-so have a turn because he's a nice old chap and we're sure he means no harm but it can be difficult not to look bored (or exasperated) when he hogs yet again.

But instrumentalists looking bored when someone's singing? I can't say that I've noticed it particularly any more than the singers looking bored when tunes are going on. Some people are just plain rude and they will talk through anything that doesn't involve them singing or playing. I don't think there are more instrumentalists than singers in this group.

Just my four pennorth!