your coil could be shorting out ie touching the walls of the mod.

Usually i get this problem when either this 2 scenario;
1) liquid creamy or metholly - cepat rosak the cotton
2) coil touching the wall of the atomizer or coil shorting out
3) your coil burn up tak sekata ( if using dual coil). One coil burn up faster and brighter than the other one. Usually after u set the coil up, sometime the cotton when inserted will screw up the coil fire rate. so end up one coil burn and the other baru warming up