The only way to kill in an LED is to overcurrent them, which drives the junction temp of the die close to 136+C and they short if white, open if other colors.
it really depends on what LED's you are running... a good set of LEDs can hold 700mA without an issue and over 1 Amp if properly cooled. A set of normal LED's usually run at 50mA up to 250mA... before they die....
I would make sure you are not:
1)over current (ing) the LED in the drivers side...
2) you have proper cooling... While they come mounted on a star board:
it might not be enough to keep them cool, and you will hit junction temps in the 140C and kill the dye... once the dye goes there is no recovery for the LED...
If you feel there are a little bad LED's go buy some top LED
Luxeon or Cree make the best LED's in my experience...
Luxeon provides the REBEL line, which has the following colors:
warm white, neutral white, cool white, PC amber, amber, red, red-orange, green and blue... they can run at 500mA on a star board without an issue:
Cree: has the XP-E line, which has all the same colors, the LED is a little smaller, and runs at 700mA without an issue:
With Cree if you just want white, you can also get the XP-G same footprint as the XP-E but runs more efficient, being a newer type of INGan chemistry.
and if you are really crazy....
go for the CREE MC-E line... its a 4 die chip, it comes in warm, neutral and cool white, as well as an RGBW chip (one red, green, blue and white die)
These things at 350mA put out 430 lumens which is about the same as a 100W bulb.
You will be paying a higher price for them... the Rebels can run about $4-5 a piece, Cree XP-E about $6-7, with the MC-E depending on bin as high as $30... but the MC-E is over kill.....
but step one, measure the Vf and Current you are supplying... most LED's will require around 3-3.5 Vf to run.