My guess is you are probably correct. You have a timing issue.
( this assumes by coiling you meant the line is just piling up on the ground or water not far from the rod tip, and not fully extending. If instead you meant the line is coiling due to memory from being wound on the line spool, then you need to learn to get most of the coil memory out of the line by stretching the line. )

My view is
- shorter lines are easier to cast than longer lines.
- heavier lines are easier to cast than lighter lines.
- tapered lines are easier to cast than level lines. Either mono or furled.
- on the back cast. Stop at 12:00, pause for a moment to give the line time to extend rearward a bit before starting the forward cast.
- make your back cast a little faster than the forward cast.
- stop the forward cast at about 2:00 or 2:30. Not an abrupt stop, like you hit a mechanical stop. Rather like a quick deceleration, like you want to stop the forward motion in the shortest distance possible before hitting an imaginery mechanical stop.
- don't start your forward casting motion to abruptly,with to much power. make the forward cast at a steady linear increase of acceleration rate.
* don't think about the last 3 points to much. Just keep them as secondary background thoughts, your subconscious will figure out how to apply them during casting practice. At least that is my theory.

(it takes time to develop fast enough line speed to cast a light line. A heavier line can be cast properly with a lower line speed than a lighter line)
I also believe the speed of the tip of a longer rod is faster than the tip speed of a shorter rod. Therefore, I think if you have a zoom rod, you will find it easier to get faster line speed with the rod at full extension. Getting proper line speed with the rod at the shorter lenght will come with practice.

I consider a short line to be a line that is about the same length of the rod or a little shorter.
And a long line to be a line that is 4 feet or more longer than the rod.

I like to watch this video of Masami Sakakibara casting a 12meter tapered line.
The time period to cast a shorter line will be shorter, but the sequence of the timing - backcast, stop, pause, forward cast, stop, will be the same.

Notice the speed of his back cast, the pause before starting the forward cast. The deceleration and stop on the forward cast. Think about that motion and timing during your practice casting.
(it may also help to do a bit of side casting during your practice. When side casting you can see the line extend on the back cast and only start the forward cast when the line has extened rearward enough. It doesn't have to be fully extended, but close, especially in the begininng. )

https://www.youtube.com/watch?v=jalWBL9iuf4



Part of the trick when the timing is correct is, I believe, you start the forward cast when the tip of the rod is still flexed rearward a little, then on the forward cast the rod tip will flex forward when you stop the forward cast motion, and when the tip continues to move forward it will put power into the line to propel the line forward to full extension.

When you can cast a short line to your satisfaction, and want to try casting a longer line. You can first try casting a longer line of the same weight or size you were using before. If you can't get it to cast properly after a little practice. Try casting a heavier line at the new longer length. The heavier line won't need as much line speed as the lighter line. For example switch from a #3 line to a #4 line.

All that being said. I also believe that trying to cast a longer line, even when your casting is not yet very good, helps you develop a sensitivity to the line, and what it is doing to the rod tip, that you just won't develop casting a short light line. Casting a longer line will magnify improper technique. And what that reveals may help you improve your technique casting a shorter line. When I thought my casting wasn't as good as it should be. I found that after trying and failing to cast a 6 meter line very well for 30 minutes. When I immediately followed that by switching back to a shorter line. The muscle memory from trying to cast the 6m line, had slowed down my cast, improved my pause timing, and my casting of the 4 m line was better. Therefore, I really recommend learning to cast a short line half way decently, there after. Practice casting a longer line for 15 - 30 minutes. Then immediately practice casting a shorter line.

I think casting in the yard is ok for a lot of your practice. But I also think its better if you can cast into water. I think the drag on the line as it is lifted from the water helps to load the rod (that is flex the rod) thereby adding some line speed during your back cast.

2? from a self taught tenkara caster.

Good luck.

D