I agree 28 gauge is a bit too thick for a Protank coil. It's doable, but you have to really be careful when threading the cotton through the coil, so as not to push the coil so it touches the side. You are probably doing that and causing a short, which results in the drop from 1.5 to 0.5 ohms.
Personally, I use either 6 wraps of 30 gauge, around a 1/16th drill bit, to achieve about 1.7 ohms or 6 wraps of twisted 34 gauge, around a 1/16th drill bit, to achieve 1.9 ohms. I have to say, right now, I'm thoroughly enjoying the twisted 34 gauge, 1.9 ohm coil, when vaping at 5.0 - 5.3 volts on my Provari. Using Green Apple from MBV, the warm vape really brings out the flavor.