Are you dry-burning the new coils, as well? If so, chances are that you are burning up the wick, which is causing your burnt-tastes and poor vapor (because the wick has reduced ability). If you burn the wick inside the coil, it cannot properly wick juice TO the coil for proper atomizing.
There is no need to dry-burn a replaceable coil, especially if new! If you are dry-burning to remove build-up on the coil (which would be non-existent on a new one anyway) to prolong it's life, you should remove the wick first, burn the coil to remove residue, then re-wick with fresh material (silica, cotton, rayon).