That is why do not wire the leds parallel,
every individual led or series led group
should be used with series resistors
before wiring it with another led or led group.
I guess one of the led in that circuit
had a lower operating voltage,
if you detect and get rid of that one
rest of the leds will start to become brighter again.
The suspected one would be the brightest one,
I don't know may be the dimmest one.
Also:
When a led gets hotter
it's resistance gets lower,
means it draws more current,
if it draws more current
it gets even hotter,
if it gets even hotter
it draws even more current
it is getting nowhere
except killing itself.
So if there is not any resistor for
limiting the current for each of the leds
this problem eventually happens.
You may also search for the hottest led in your circuit
to find the guilty one .