A few things to check:
1). Make sure the lead wires are clipped as short as possible. You should be able to run your finger around the base of the coil head and not feel any excess wire poking out. If the wire catches your finger, there's potential for a short. This is why most shorts occur. I use fingernail clippers to really clip the leads back into the head as far as possible.
2). If your coil is too high, the stem/post could catch it and touch one of the sides or the top of the coil. Ideally, you need to make sure all coils are as low as the stock coils. Use a stock coil head for reference.
3). If your coil is not evenly situated, the leads in the well could be touching the sides. It's important to keep those straight.
4). Make sure your coils are even. Use a drill bit, paperclip or small screwdriver to keep them even. A 1/16" drill bit or allan key will fit in the cut outs of the protank heads perfectly, so you can actually keep the drill bit or allan key in there while you assemble.