I'm certainly not an expert wheel builder, but have now built a half dozen or so wheels. My only thought is that on my first few attempts, I didn't get it 100% right on the first try, so I had to take the wheels off and re-adjust (or in one case loosen the spokes and start over) a few times.
As such, I might suggest starting with an innertube until you're really happy with the wheel, unless it's easy to remove and replace a tubeless tire.