I had a discussion with my manager about this the other week and they asked me to let them know if I was considering other work so that they could try to fix it - my response was that I didn't think that would help. By the time I'm considering another job it's _because_ I no longer think my manager can or will do anything to fix the issues I have.
People generally don't leave companies when they feel like their manager would have been able to solve their issues with the company.
I can't read minds. I need people to tell me what works and what doesn't for them. An employment is a continuous transaction, if the situation changes it is absolutely open to re-negotiation. It's on both the employer, the direct manager and the employee to make sure everybody is happy with the arrangement.
And the two times I did hear about problem before the new contract was signed, I was able to find a solution. In the other case there is not much I can do.