This belongs in the RNG Help thread, but I'll answer it here:
Most likely it's because what you're looking for is not catchable in the game with your SID, the required IVs and the nature you have chosen. Caught shiny Pokémon with high IVs are much more difficult to obtain than non shiny ones because your SID dictates what IV and nature combinations can be caught shiny and which ones can't. Perfect IV caught shinies are usually caught by people who RNG'd their SID to allow this to be possible, though I have seen people who had perfect shiny spreads through sheer luck. Try breeding instead. Or, if you absolutely must catch it, lower your IV requirements.
Thanks, you were right. I had to adjust some of my IVs to get a shiny spreadsheet.