the other day i bought 6 led s in a packet for the new station lighting and on checking them with a led tester they all passed muster and worked ok so i then added the wire tails and ran the wires back to the resister circuit board as per my normal procedure two leds to one 1k resister i then checked all the solder joints and eyeballed the whole thing over just to make sure all was as it should be . but on switch on only one led would work so every thing was stripped down and checked again but on re assembly i got the same result after some head scratching and 4 hours later it transpired that one of the leds would only work on its own but when connected into the system would cause all the other led s to switch off now as they are all wired correctly and when the offending one is removed every thing works fine can any one tell me why please as ive never had this problem before