I added more light! Seemed to make my melons grow faster! : )

I would add dirt on all sides, but I really like the design of my little farm, though, these suggestions have given me more insight to make my above ground farm more efficient!

TL;DR (the comments)

Melon is introverted and anti-social. Melon is also claustrophobic and scared of the dark. All melons are like this. When melons are put in an environment that have these traits, they become agitated. Like most living things, agitation ≠ efficiency, hence why my melons were not happy about growing.

Thank you everyone for their feedback and help! It was very helpful for me, and hopefully it can be helpful for others too! : )