Even though it can be vastly varied; from lullabies to synth funk to thrash metal and beyond, most stock music obey to some unwritten laws. That’s because, generally, it must be functional as background for a wide range of video projects (sometimes with voice over the music too). So, one of the reasons for your rejection might be that there are very busy swirling solos in there, hovering over wild rhythms twists, etc.
Just have a listen to a few categories to get a vibe of what has better chances to be accepted. Because, with your level of musicianship, it’s probably just a matter of adjusting your style to what has proven to sell steadily around here.