It is so important for a reader to form an attachment to the characters right away. You may have a great plot, but it is the characters that are going to carry the story, it is through the characters' eyes that the reader will view the world you're creating. If I don't care about character, or what happens to a character during the course of the novel, then I am less inclined to continue reading. I'd like to share a few thoughts about connecting with a character.
1. I will often find that a character feels "stagnant." That is a character(s) will stay the same throughout the story. The MC needs to have goals and motivations, and those goals/motivation need to realistically reflect the personality/voice of a character. Why are they doing what they are doing?
2. The voice for each character should be different!!!!!! For example, a grandmother shouldn't sound like a teenager (that might be an extreme example, but you get the idea). Also, if the voice is too similar for each character, they become almost indistinguishable. The voice should reflect: age, gender, education, and social/economic background...pretty much the voice should show us almost everything about a character.
These are 2 of the biggest things I've noticed, and I hope you'll find this post helpful!