I chalked my first YA novel up to experience but only after I made it to the point that two (out of seven to whom I sent queries) agents asked for the complete manuscript. One of the agents never wrote me after requesting the complete manuscript--I just had to assume she/he was declining, based on no response.
My question: I am now getting ready to query a second YA novel. Can I start at some better place with these two agents--mention that they had expressed some interest in my first? Or do I just start from scratch on their agency web sites, since they didn't like the first enough to sign me? I am especially leery of the one who did not respond, although the correspondence up to the final interaction was cordial and even enthusiastic, and he/she is a reputable agent from a respected agency.
Sure, what the hey. If they liked your work the first time around, mention it but don't bank on it. It might help, it might hurt, but in the large scheme of things, there are so many agents out there that if your book is good, someone will pick it up. Probably.
As to people who get to fulls and don't like them, not responding is rude but sadly not uncommon. My boss requests a lot of fulls but very, very rarely accepts one, especially in fiction and usually, she says, because she really likes the writing or subject (the reason she got past the partial stage) but didn't like the ending. It's surprising hard to bring a book to a good ending. I know I was rejected for many years for "structural problems" and still have that issue with some of my work. So the agent might say, "Hey, this is a person whose writing I liked, but the last novel just didn't work. Maybe experience has gotten them somewhere."
Or they may not remember you at all. There's always that possibility.