I'm not sure, sorry. I used to return manuscripts with a standard reply almost always as time so short and my only suggestion is to make sure you're choosing agents well to make sure they deal with picture books/ specific age group/ whatever you're sending in. The Writers Yearbook is helpful here I think.
Also maybe get feedback from a friend if helps? Hope you get more advice and hope it goes well
I find that some give feedback if they think your work is good, but most don't have time. The only way to get decent feedback is to join a critique group, pay a consultancy like Cornerstones or do a 1-2-1 with an agent at one of the writing festivals.