hmm never heard of solder being that crappy..... i mean cheap solder is pain to work with but you should not notice dim LEDs due to solder as that would mean the solder has resistance that is limiting the current to the led.
do some simple tests to prove to yourself it is not the solder by solder some wire together and checking the resistance with an ohm meter. it should register as less then 0.5ohm depending on the quality of the meter. If you see a resistance of like 20 ohm or greater then your solder is bad and likely expired.
I would be willing to guess that you are limiting the LEDs by not having enough forward voltage available. espeacially if you are using blue leds with no limiting resistor on say an xbox wireless controller, then the problem is your LED "knee voltage" on this batch of LEDs is higher then what you were using previously.
Example: if you use a LED in the range of 20mA at 3.0V and it words sweet, an led from the same batch or different batch could be 20mA at 3.1V in which case you would notice that LED being dimmer then the previous one.
Always make sure to order you LEDs with the lowest Vf possible espeacially with Blue if you only have 3.0V available
