I have several fulls out (holy hell) with respectable NYC literary agents (no, you're not one of them because you don't rep my genre). In the last week, two agents rejected - but with very positive feedback. The few criticisms I received could be fixed with an easy revision. This makes me wonder: what prompts an agent to ask for an R&R versus rejecting it entirely? If you can give some perspective on this thought process - it would help me sleep better. Well, that and a nice 20-year old Scotch.
I'm not one of them because very few people would call me respectable (barroom floozy that I am) Respected might be the better word for what (other) lit agents are.
But I digress...
I can only speak to my own practices here and I really don't know how other agents do this (it's not a topic at the DisReputable Bar and Grille where I hang my hat at 5pm)
I ask for a revise and resubmit if there's a structural problem that can be fixed. Examples of this are plot holes or inconsistencies; lack of chapters (my newest client had this!); a more compelling ending.
Structural problems that need a general overhaul generally are passes. Examples: no sense of the protagonist's code; the plot didn't have any kind of twist or surprise element; lack of a narrative arc (more common in non-fiction than fiction); lack of tension.
As to your situation: you don't know if the "few criticisms" are the extent of the problem. When I pass on a manuscript I'll generally give the writer an idea of why, but it's NOT an editorial letter that lists ALL the things that need to be revised.
This can lead to authors thinking if they fix the "few criticisms" the agent will then have a novel ready to go on submission. That's almost never the case.
Instead of trying for a revise and resubmit here, look at the feedback you have. Is it consistent? If so, that's definitely something to fix.
And read the manuscript with fresh, critical eyes. Does it have twists? Can you identify the main character's code?
But mostly, keep querying. I've signed and sold things that other agents didn't want.