Ok here's something I've wanted to bring up for a while:
Why do people remove the carb cap to clear the piece? When the vapor runs out, it will still clear just the same with the carb cap left on the whole time! All you do removing the cap is trick yourself into thinking your dab has finished evaporating for a moment until invisible trails become visible and you realize you have lost half of your dab!
Pretty much every single video I have ever seen of someone dabbing on an enail has this happen and they lose heaps of the dab.
Now onto the possible sources of your trails:
1. You may need to jack the heat up, on a competent enail rig if you are getting trails you need to raise the temp somewhat.
2. The nail you are using has poor heat distribution, leading to pooling of your dab and very slow vaporization at certain points on the dish and leading to your trails.
3. You may simply not be hitting the piece for long enough!
4. As above, you may not be carbing long enough and there is not enough pressure in the dish to push all of the vapor down through your piece.
5. Your carb cap may not be very well designed, the hole in the carb may be bigger than the inlet hole on your nail for example, this will cause your vapor not to flow properly through the piece and want to go in the reverse direction. Your carb may not also sit properly on the nail or have various other potential design flaws.
Hopefully these possible issues help you work out your problem brother, happy (lack of) trails!