Could be a number of issues, so going from the top at the tank down, I'm not a big fan of Smok products FYI, but these are the areas I'd look at...
1) You say you are priming the coil head before hand. That is not all you need to do to prime a coil head properly
A) After saturating the exposed wicking and down the center of the coil head, you still need to let the coil head soak in the tank with juice in it a few minutes, 50/50 to 40PG/60VG that time can be about 5 to 10 minutes, 30PG/70VG up to 15PG/85VG could be 15 to 20 minutes due to how thick the liquid is, thicker VG liquids take time to wick properly, as VG wicks very very slow.
B) Once primed and let to soak, cut off the airflow completely, without firing the mod take a pull on the tank to create a vacuum, then quickly open the airflow as you are pulling. This creates a vacuum, and sets things up allow proper negative pressure to force efficient wicking
C) Set the wattage below the minimum wattage rating of the coil, example, coil rated at 50watts minimum, 75watts maximum, you could set the watts to about 35 or 40watts to start.
D) In short, gentle vapes, say under 5seconds duration, start vaping, after about 10minutes continuous use, up your watts and take a little bit longer pull, maybe 6seconds max, rinse and repeat this step until the vape gets to your preferred flavor, vapor output, and throat hit sweet spot. Just like the motor of a new car, or doing a rebuild on a motor you have to gradually break the motor in each time, the coil head is the motor of your vape, its what does all the work, the wicking can be compared to the radiator cooling system of the motor, you have to gently heat up the internal thermostat to make it open up properly to provide maximum liquid movement, the wicking is the thermostat of your coil.
E) Caveat and warning - any time you get a harsh, or feeling to get dry hit, back your watts down to the previous setting, if you get a burnt hit, take out the coil and replace, start at Step "A" again. Not all coil heads are made equal, nor is every vaper identical, you don't need to run a coil at its recommended ratings, Smok especially is notorious for coil heads that perform under par, especially if not broken in properly, also remember, juices play a "Very" big part in what the final settings will be on a coil, the ratings on a coil are just averages, not gospel and written in stone. Some coil to juice to user combinations can be almost 10 watts plus or minus on either side of a coils ratings once all the pieces are in place, some juice flavors love higher watts (bakery flavors as example) while other love lower watts (straight fruit flavors for example). Vaping is not plug and play and go on with your business, a lot of variables have to align just right to make the perfect vape for any vaper, though new technology can get pretty darn close if you use the same flavor juice every time.
2) The connection between the tank and the mod could be finicky (just barely making connection) making the Ohm reading of the coil unstable, or the coil in the head could be unstable. This Ohm instability or improper reading could be making the mod push higher voltage than needed, thus increasing your watts above what is needed. Example, a 0.25ohm coil rated at 50 to 60 watts could theoretically need about 4.2 to 4.4volts to reach those watts, but if the connection is unstable, one moment the mod is reading 0.15ohms, but the next moment the mod re-reads the coil when you fire again and gets a reading of 0.35ohms, the mod will push higher watts at that 0.35ohm reading where before it was a much lower voltage needed for the 0.15ohm reading. Remember, these newer mods read the Ohms of a coil hundreds, even thousands of times a second or minute, especially with a mod that is capable of Temperature Control, they use that reading to guess the temperature of the wire when set into TC mode, higher the resistance of the wire, hotter the wire is.
3) Speaking of TC mode, make sure the mod is not set into one of those modes, there are 3 types of wire that work in TC mode, Nickel Ni200 (TC Mode Ni, Ni200, Nickel), Titanium Ti1 (TC mode Ti, or Ti1, or something to that effect), and Stainless Steel 316L (SS316, SS316L, or Stainless Mode). Ni200 and Ti1 should only ever be run in TC mode to begin with, SS316L can be run in either TC or Straight Power Mode, Kanthal A1 and NiChrome (NiCh 60 or NiCh80) are only to be used in Straight Power/Wattage Mode. Make sure the mod is set to the proper mode for the proper wire the coil head is made out of.
In honesty, I think your problem lies in either #1 or #2 from the sounds of it. #2 can be figured out by using another mod beside a H-Priv, use a friends or such to test if you get a stable vape on the X-Baby, if still doesn't work properly, the problem lies in the X-Baby tank, X-Baby coils.