In general, you don't respond to rejections to ask questions. (Even if you're polite, the folks most likely to do that tend to be stalkery/obsessive/abusive types, and you don't want to suggest to an agent that you might be on the verge of mailing a box of cat turds to his office because he didn't think your book was marvelous.) It's really not about what you do or say in asking for feedback, as it is about what they might be afraid you'll do or say based on the small percentage of bad ones out there.
Also, form rejections are always disappointing and it's easy to think the agent was being careless or lazy by not saying anything detailed. (Not that you are saying that, but I see that attitude fairly frequently from folks who are querying. "They took a month to read my full, the least they could do is give me notes on my MS." or the like.) But, if they aren't giving you anything other than a form there is a possibility that there isn't anything they can say that would be constructive. Some manuscripts are just fine, but they just don't grip an agent in a way that makes them think they could represent it enthusiastically. And trying to come up with a "reason" could cause you to change something that really doesn't need changing. Or that might have made that one agent a little happier to see, but isn't a flaw, just a personal preference.
Best of luck as you continue querying. It can be a slog sometimes, but be patient and hopefully you will find that agent who really loves your book as much as you do AND can put it in front of the right editors.