The rod you describe is a soft rod with a lot of flex. From your description, I think you are waving the rod as in Davo's first illustration. This is the classic beginner's "windshield wiper cast".
Although I agree with Davo's first illustration, the second is bit misleading. A level stroke as illustrated will not
give you a level rod tip path. The reason is that a fly rod is a flexible
lever and as it shortens, the ERL (effective rod length) shortens. The fly rod tip actually comes closer to your casting hand. So the actual path of the rod tip would curve down toward the hand. This will form a tailing loop.
The illustration I like better is the one below. It represents the shortening of the fly rod and the compensatory curved path of the casting hand. It also shows how the rod shortening increases as we make longer casting strokes an the stroke path lengthens for longer casts.
In your case, I think you are basically waving the rod back and forth without the hard stop that is needed at the end of each forward and backward cast and the delay that is needed for proper timing.
Rather than try to explain in words what is needed, here are a few videos from a local fly shop in my home state.