advice is to remove the barreljack connector and instead use electrics clamps or solder the joints, reason is because of resistance barreljack connector gets hot, less current can flow voltage will drop.
- Make sure gauge of wire used is enough, you can make a calculation about the thickness/gauge AWG18 is not thick enough for 300 led feed.
- make sure you save the ledconfigurationpage correctly so the leds will be there in the Json file.
- Make sure grounding (GND) is connected to PI gnd pins/PSU gnd/ledstrip
and also power the ledstrip from BOTH sides, so beginning and end needs +5volts on VCC and GND on GND