The sketch you posted has no code to receive or action any IR commands. So it won't "acknowledged" anything. Did you post that sketch by mistake? It doesn't help us diagnose why your other sketch, the one that receives IR commands, works with 30 LEDs but not 400.
But it may be that because updating a longer strip takes longer, parts of the IR signal are lost. If your sketch updates the strip too frequently, there may be no opportunity for it to receive the IR signal in full.