My latest manuscript came really close to getting an agent--I had several partial requests and a full manuscript request. Each agent that read my partial gave the same reasons for rejecting it, so I know what the problem is. I was wondering: when is it appropriate to revise and resubmit? (I was planning on taking 4-6 months to revise and make sure it's fixed, polished, and perfect.)
Absent the specific words revise and resend, never.
This is one of the main reasons to not query all agents at the same time. If you do get requests for partials or fulls, and you do get feedback on changes, if you've queried everyone, you've got no place else to go.
There are a gazillion agents out there though. Make a new list and query when ready.