I may be way out of line here, so ignore me if I am.
One of the things that can make a PB sound old-fashioned is if the author explains things that the illustrations can or do show. Years ago, the author wrote the story and the illustration was sort of a bystander. Now the text and illustrations work together. For instance, if the MC is wearing a red dress, it would be old-fashioned to mention the red dress because the illustration shows it - unless, of course, there is a really good reason to mention it.
Anyway, I know this is pretty basic, but I thought I"d mention it.
Good luck. I know my current first chapter has been critiqued by three agent/editors. Two liked it, one didn't. The one who didn't reviewed the chapter as if it were a thriller (she used the word), when the ms is actually a cozy mystery - big, big difference. She obviously missed the point, so maybe that's what happened to you.
Laurel