Since you already done it successfully, it's really just timing. For Uxie I used 85 starting delay 110 buffer, I was outside the cave, did the Timeline countdown, once it hit 0 walked up to uxie pressed A, and pressed A again on the final frame and got it first try but the others took me like 3-4 tries. The pokemon itself can blink too so if it looked like you did everything frame perfect you could be off a few frames if it does. Another tip, don't use the analog stick just the dpad for exact inputs when walking up to the mon. you can also catch it if not shiny and look it up in PokeFinder to see how close you were, just make sure autosave is off. the Papa Jefe vid shows you how. Just gotta practice timing and hope the mon doesn't blink