Help with LEDs in parallel
Those string lights have tiny wires with considerable resistance. It is unclear how you are attaching them, but if you are wiring them sequentially in parallel, the voltage drop along the wire results in much less voltage at the far end of the 10m line.
There are two options that I can see:
Run a separate, lower gauge wire to each strip and posibly to both ends of each strip so that current doesn't have to pass through the more resistive strip wires.
Wire the strips in series and connect them end to end. This will require a 15v power supply.