"couldn't connect with the MC's voice" could really be ANYTHING. The agent I'm interning for often gives this as a reason for turning down MSs, and the real reasons have ranged from:
-MC is too abrasive to other characters, thus making him unlikable.
-MC is too quiet, so she became kind of boring to read.
-No real character arc to MC. He does not grow throughout the novel.
-MC is a terrible person, but his terrible characteristics (e.g. narcissistic, spoiled, unreasonably demanding) are portrayed as positive things.
-MC is a 12-y-o girl who sounds like a 30-y-o adult. This could work if the MC is meant to be precocious, but in this case, the author just did not grasp the character well and we felt it wouldn't appeal to young readers.
-MC is a child who is too evil, thus making the MG book too gory for young readers.
-MC is too unpredictable and shrieky, making her incredibly annoying.
And so on and so forth. It's a throwaway phrase that you can't really glean much from, unfortunately. :-/