I have to ask, when setting up a new coil head are you properly priming it?? 5-6 drops in the center, then soak the juice inlet ports, install into tank, and let the tank a least set for 5-10minutes before use, thicker juices the soaking time is longer, or another option, have a coil head already soaking in your bottle of juice after you buy it, just need to clean up and out excess juice off the contact pin and out of the air shaft sections.
Known things that clog up coils-
1) To high a power setting, causing the wicking to scorch one it gets to dry - 20watts you sound fine on this
2) Thicker, higher VG juices will wick slower, if the wick can't keep up with absorbing liquid and power set to high like in section 1, wick gets scorched. Also, VG will carbonize and collect on the coil faster than PG will, it is also sometimes a sweetener, see #3
3) Sweet liquids due to added sweeteners like sucralose and others will carbonize and crystalize onto the coil and in the wicking near the coil, also flavors like caramel, custards, bakery, candies, milky flavors, cream flavors, coffees like caramel machiato, vanillas, and others in this category will do the same (some excessively sweet fruits will do this as well)
4) Colored liquids, clearer the liquid, less residues are in the liquid, though when the liquid is very dark like coffees and tobaccos, any with food colorings in them, will not vaporize off easily, collecting as gunk on the coil and in the wicking next to the coil
5) Tobaccos, especially Naturally Extracted Tobaccos (NET) and Whole Tobacco Alkaloids (WTA) due to color and extra particulates in the liquids due to their extraction processes, will collect gunk on the coil and in the wicking next to the coil
/edit
Going to ask this as well, what airflow setting do you have set for the coil head at 20watts?? Full open, 2nd smallest, or smallest??