Like mentioned above, there is probably build-up on the coil, not allowing the coil to heat evenly and possibly creating hot spots and even burning the build-up on the coil (all creating a rough sensation). This will be noticeable as the wick starts turning dark. A coil replacement is recommended. Other possibilities would include: water has got into the liquid or coil which causes poor performance, power on the device is turned down to low, airflow has been restricted or cutoff or blocked, or too high of a nicotine strength is being used causing discomfort leading to small weak inhales.