Most candle problems have the same two causes: the wick is too long, or the room isn't as still as it looks. Here's what's usually going on.
It's smoking
A thin trail of black smoke almost always means the wick is too long, or there's a draught bending the flame sideways so it burns unevenly. Trim the wick to about 5mm before every light, not just the first one, and move the candle away from windows, fans, or a doorway that gets a lot of foot traffic.
It's flickering constantly
Some movement is normal — it's a flame, not a light bulb. Constant, dramatic flickering usually means a draught again, or a wick that's started to curl over to one side. A quick trim and a steadier spot usually settles it.
The flame is sitting low, almost drowning
This is a wick that's gone too short relative to the wax around it, usually from over-trimming or from tunnelling early on (there's a whole post on that, if you missed it). If the flame looks like it's struggling to stay above the wax line, either fix the melt pool with a longer burn, or, if it's near the bottom of the jar anyway, just let this one go.
When to actually stop
Once there's about a centimetre of wax left at the bottom, or the jar starts feeling too hot to touch near the base, that candle's done its job. Squeezing out the last bit isn't worth the risk.