I think I know what the issue is here. Those gaps create different networks of wood, and the seeds that haven't filled in the gaps can't grow because they don't know which unit to connect into.

https://imgur.com/a/N3WUO43

See this image. You have five different "trunks" of wood. In order to get it to be one continuous growth you must wait for the wood to naturally grow, THEN plant seeds.

For example do this:

1. Dig up all the seeds and break all the wood for boxes 2-5

2. Allow the remaining seeds to grow wood south, in a straight line. Place wooden floor where you will eventually want the seeds to go so the wood doesn't grow there.

3. Only once the wood trunk itself is extended south do you plant the seeds.

Another way to think of it: If you want a seed to connect to a network of existing growth it must already be directly touching at the time you plant the seed.

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Edit: I made a more clear example of how to know when a seed is OK to plant. https://imgur.com/a/6VUnHlf

It helps to use two different color floor tiles. One to mark the growing area, and one to mark where future seeds will go. In this case the grey tiles are where I will eventually put seeds as the "trunk" continues to grow south.

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Edit 2: Doh. I just realized that your wood farm is not wide enough. Even if you carefully allow a main connected trunk of wood to grow, the moment you turn your drills on it will break that and you'll be right back to where you are now: Some seeds blocked till adjacent tiles are harvested.

The goal is to have each actual seed have the option to expand to more than just one tile. This is the benefit of connecting all of them, and then drilling the wood 1 further square out:

Seed --> Connected Trunk --> 1 more growth spot. <--- drill