Sooting
Updated
The complaint that arrives as "your candle made my wall dirty", which is a serious complaint and usually a true one.
The black ring
A dark band on the inside of the vessel above the melt pool, appearing within a burn or two.
A trace after many hours is ordinary combustion. A visible band after one four-hour cycle is a fault.
Where it goes
Not just the glass. Soot from a badly burning candle deposits on the ceiling above it and on nearby surfaces, and a candle burned regularly in the same spot leaves a mark.
That is the version the customer notices, weeks later, and it is unrecoverable as a relationship.
During testing, look up. The ceiling above the test candle tells you what a customer's will look like in a month.
The causes
Incomplete combustion, which means too much fuel for the flame: an oversized wick, a load above the wax's capacity, a heavy dye.
A draft, which disrupts the flame and is the only cause that belongs to the room rather than the candle.
A long untrimmed wick, which is the customer-side cause and the reason the instruction exists.
What to do
Drop a wick size first. It fixes most cases.
Then the load, a point at a time.
Then the series, if a smaller size tunnels.
And check your test conditions before changing anything: a test run near an open window has been measuring the window.