Often the voltage drop along the strip can be significant. I have a lot of strips where they don't come anywhere near their rated current if a long section is fed from one end (in my case 5 m of 1.2 A/m rated strip drawing about 2 A rather than 6 A), but if you cut just a short piece it draws the expected current.
Try measuring the voltage across one of the 150R resistors near the feedpoint, that will give current for that group of LEDs, and then another one near the other end. Worth also measuring the voltage across the far end and seeing how much less it is than at the source.
Possible mitigations include feeding from the middle, feeding from multiple points, but the best one is to feed from opposite ends with a constant current supply (positive in one end, negative in the other, the supply can then wind the voltage up to overcome the drop without any groups of LEDs getting overrun).