I don't have good luck with models.

[Michele Rushworth]

I think you need to keep trying different models. Clearly the talkative model that you described won't fit the bill, unless you can help her to understand what is required. The other model, the sleepy one, may have just been having a bad day after being up with her baby all night. A full day of sitting still may not be a good choice for her. Maybe shorter sessions with her would be better.

Also, if you want other artists to share the costs with you, I think you are well within your rights as the host to ask people to keep the chatting to a minimum and that the purpose of the get together is to create an environment where people can concentrate on their painting. Perhaps you could add on a social hour at the end, if people want to do that.

Maybe take a hiatus for a short while and then re-start with the new "rules". Don't give up!