Probably too much tension on the wire.
What I'm going to outline here only applies to single-core claptons and will not work for fused (well, it can, but its extremely difficult.) All the same, I do encourage you to try it. This method was a huge breakthrough for me when I discovered it. It took me from getting a decent wire 30% of the time to getting perfect wire every time in the time it took to try it. If you can get it down, then you will be making perfect single-cores every time with ease. You don't need swivels or nothing. Just your wires of choice and a drill.
I unravel maybe 3' of the outer wire, re-secure it to the spool, tie it to something heavy or attached to the wall, release the wire from the spool with it pulled taut, take the spool, and walk it back about 20-30 feet. This should be enough to do roughly a foot of clapton wire. From there, I very, VERY gently start applying a steadily increasing amount of tension until I actually feel the wire give and stretch by maybe an inch or two. Just that initial give is enough - you don't want to snap it and too much stretching will make it much thinner than what you started off with.
This simply straightens the wire so that it can just sit off of the spool without tying itself in knots. That nice, slacked and loose wire is vital to this technique.
The idea behind this is that when you're making a standard clapton, the only reason you need any tension on the outer wire is to keep it from flying wildly off of the spool. Everything else about that tension is actually working against you. If the angle's just a little wrong, your grip falters, the outer wire shifts/recoils, or the spool hits a little snag, you're sunk. Any shift in tension threatens the integrity of the wire. All of that constant pulling and positioning makes the wire harder to control. What you're essentially doing is pitting your hand up against the torque of the drill, which can and will either overcome your grip and make the wire jump or snap the wire at the chuck... ...depending on which is stronger between the wire and your grip.
With the outer wire straightened and resting completely off of the spool, all that you have to do is guide the wire with very minimal pressure and no tension. Eliminating the need for tension takes the work away from your hand and gives it to the drill, which is more consistent and reliable than your hand ever could be. Just hold it in place and watch the magic happen. You don't want to fight it or have it fighting you - let it do its thing.
The hardest part is finding the right angle at which to pinch the outer wire right up against the core. And I do mean RIGHT up against it... ...as in, you want to grip both the core and the outer wire between your thumb and index finger. The wire should move freely past your fingers. No friction or resistance. It should feel just like you were taking the core wire and slowly sliding your fingers across it.
The best way I've found is to let the core sort of rest at the point where your thumb and index finger meet and let that also be the point where the outer wire first meets the core. I personally find it easier to pull off consistently with the drill running clockwise and thumb parallel, as the thumb is easier to keep secured, but you can also go counter-clockwise and guide the wire with your index finger for a little more control.
This is purely a matter of which finger/direction you're more comfortable with having as the dominant one in your grip. You'll know which direction works for you because one will be significantly harder than the other. For me, it actually varies from week to week.
It's tough to find the right angle to come in at - I could show you but it probably wouldn't help, though once you do, it's a lot easier than trying to actually guide the outer wire while holding tension on it a 1/16th to an 1/8th of an inch away. It's got nowhere to go between your thumb and index finger but the groove you work it into. The key here is to relax your hand, wrist and elbow. If you pinch at an awkward angle or find yourself needing to pinch it tightly, then you're not holding it right.
That's how I've always done it, anyway. Good ole freehand style, heavy emphasis on the 'free' aspect. Your finger and thumb are just there to guide the wire into the groove and keep it there. After that, you let the drill do all of the hard work and the wire sort of wraps itself. It even seems to guide itself and in fact pushes your hand along the core as it goes. It feels completely effortless and automatic. Once you know how to do it, you can make perfect single-core claptons without even looking down at the wire.
