LEDs are great in terms of their rated life expectancy and meager power usage, however, as diodes they have by their nature an inherent voltage drop as the current flows through them (about 7%). So it is easy to understand why it will be an issue to have a series string of LEDs with diminishing voltage at the end of the line with every diode.
Some diodes have a very narrow voltage range, so at some point in the series the diode will no longer emit light. LEDs are also adversely affected by voltage fluctuation, and high and lows will shorten the life expectancy of the LED which is why some more expensive fixtures have a ‘regulator’ built into them to stabilize the voltage.
The best advice is to not wire LEDs in series, and if it not too much of an obstacle run a new circuit for the additional lights.