The dim end is physics, not a defect
Walk along a 20-metre curtain run on a winter evening and you will notice it: the first third is sharp and bright, the last third is soft and grey. Most buyers assume the LEDs at the end failed. They did not. The driver at the plug end is pushing a fixed voltage, and every centimetre of wire between that driver and the last LED steals a little of it. By the time the current reaches the far end, the voltage available to light the LEDs is lower than the value printed on the spec sheet. That difference is the voltage drop, and it is the single most common cause of uneven brightness on long strings.
Where the voltage actually goes
Every LED in the string sits behind a small series resistor, and the whole string is fed through copper wire. Copper has resistance, and resistance scales with length and inversely with cross-sectional area. A 20-metre run of thin wire carries current for the full length before any light is made, so the drop accumulates continuously rather than in steps. The LEDs near the driver see the full output. The LEDs at the tail see whatever is left over. Nothing broke. The math just ran out.
This is why the problem gets worse when you add a second string to the same driver, or when the ambient temperature drops and the driver sags. The load doubles, the current through the shared wire doubles, and the drop roughly doubles too. A string that looked fine on a short bench test will dim visibly the moment it is deployed at full length outdoors.
The AWG number is the spec that matters
Cable gauge is the one line on a quotation that directly controls this behaviour. Thicker wire, lower resistance, less drop. For long runs the buyer should be comparing the gauge of the backbone cable, not just the length or the colour temperature. Two strings of identical length and LED count can look completely different at the far end if one uses a heavier backbone. If a supplier will not state the gauge in millimetres or AWG, that is a reason to treat the brightness claim as optimistic.
A practical rule of thumb for mains-fed strings: the backbone should be at least 0.25 square millimetre for runs of 20 metres or less, and heavier for anything beyond. For low-voltage strings the rule is stricter still, because the headroom between the driver output and the LED minimum is small, so the same absolute drop takes a larger bite out of the light.
The driver is the other half of the equation
Wire drop only becomes visible light loss if the driver cannot compensate. A driver that outputs a fixed 12 V into a long, thin string has no reserve. A driver that is sized for the load and outputs a slightly higher internal voltage, or a transformer that accounts for the line, gives the far LEDs their share. When comparing drivers, the question is not only the rated output but the regulation under full load. A driver that holds its voltage with a full string attached is a different product from one that holds it with half the string attached.
How to audit a quotation for this failure mode
Three questions separate a string that will stay bright from one that will fade toward the tail. First, what is the backbone gauge in square millimetres or AWG. Second, what is the driver output voltage and what is the maximum load per driver in metres. Third, is the rated brightness measured at the far end of the declared run length or at the plug end. A supplier who answers all three with numbers, rather than adjectives, is the one whose brightness you can actually plan a display around.
None of this is exotic. It is the difference between a string that is specified and a string that is decorated. The LEDs were never the weak point on a long run. The wire and the driver were.