I've been using the Baby Beast for about a year and the Big Baby for a few months; it provides a stronger flavour.
I want to reassure anyone experiencing problems that they will go away with a bit of experimentation and advice. This is a great forum with some very helpful people on it.
If you're experiencing problems, don't let these tanks run too low. It is better to have plenty of liquid in them, preferably covering most of the area of the wicking holes. However, when everything is working well it does not seem to matter much if you let the liquid go down to a low level.
As OlderNDirt wrote, when screwing in a new coil be careful to make sure you don't get it cross threaded.
Check that you are using authentic Smok coils. Use the scratch-off portion of the packaging and check the code on the Smok website.
Sometimes you may get a duff Smok coil, but that is very rare in my limited experience. In a year of use I have had one coil burn out, while inhaling, and the taste of burnt metal is not something I will easily forget. I've had one or two other coils that failed within a day or so.
I'm not the biggest fan of the Q2 0.4 ohm coil. If I remember correctly (and I may be wrong as I haven't used it for a while) it is one of the coils that can have a tendency to pop, crackle and spit with some liquids.
After trying all the different coils, I have settled on the Q2 0.6 ohm coil; unfortunately it is sometimes not the easiest coil to find, but it is worth hunting for. It is super smooth with my DIY liquid, has a lower power requirement for extended battery life and less heat, and is a cooler vape (which I prefer). It allows me to take many long hits without excessive heat build up in the mod and drip tip. I generally use it at a maximum of 32 W, going up gradually from 20 W with a new coil.
I replace coils after they have consumed 140 ml of liquid - about 2 weeks. My current liquid is 30% PG, 65.6% VG, 4.4% water.
I put a new Q2 0.6 ohm coil in the Big Baby today, primed it, filled the tank, waited all of two minutes, closed the air hole almost completely, sucked gently but continuously on it a few times, re-opened the air hole, then started vaping at 20 W, gradually turning the power up 1 or 2 W at a time. A few hours later I was using it at 32 W; the vapour is warm on a very long hit, but never gets hot.