Your wooden rectangle box is good. I can't tell from the over the top view of your circular pots. Although I don't think those are the ones in question.
I asked for more pics to get a depth size of your planters. Squash like to grow roots, 2-4 ft deep. Also, they like to shoot their roots down first, and then grow above the soil!
My zucchini had only 1 true leaf for a long time. But because I had it in a clear container I could see the roots shooting down, down, down. Now that it hit the bottom (and it's wrapping back up and around), the top is growing like crazy. This could be the case with yours. Or maybe you have a slow grower. As long as you give it what it needs, and don't over do it, the plant will kick in to another gear eventually. Your plants look really good!
If you don't mind me asking, what's your watering routine looking like now?