(1) Parents oftentimes seldom appreciate the unique attributes, personalities, & talents of their middle children. Instead, parents compare their middle children to their oldest &/or youngest children, making the middle child feel quite inconsequential, even inferior.
(2) Parents tend to understand the oldest & youngest children more because such birth orders are considered to be more mainstream & less ambiguous. Middle children are oftentimes misunderstood, even miscategorized by their parents as the "other" child in the family. Middle children can be marginalized because parents really don't know who their middle child really is.
(3) Parents tend to overlook middle children, feeling that they are independent or they feel uncertain as to how to approach him/her. Many middle children feel left out as they see their parents spending more time w/either the oldest or youngest child rather than w/them. Also middle children feel as if they are the odd children out in the family. They feel that they are forced to navigate the familial waters & to become independent of their families early because they don't receive what they need from the latter.
(4) Parents playing favorites w/either the oldest &/or youngest child to the detriment of the middle child. Studies show that middle children are oftentimes the least favorite child in the family. Studies furthermore confirm that favoritism has deleterious/damaging effects on the least favorite child. It makes such children feel as if they don't matter in the familial scheme of things. It also creates low self-esteem.
(5) Parents treating the middle child & his/her accomplishments as old news or adopting an attitude of seen that before. Middle children are special too as oldest & youngest children are. However, they aren't treated as special but as secondhand news.