Google Earth engine (GEE) -- when we use the front and back images to make up for the interpolation effect, what if there is no effect?

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This code is when we interpolate the image to make up for the cloud removed image , Most of the time, images can't produce results , So we need to change the time of our interpolated image , This increases the time of the image , So that more images can be used for interpolation . Code ：

// define Dates
var startDate = ee.Date('2003-01-01')
var endDate = ee.Date('2021-12-31')

// Define another mask function to create mask from SmmaryQA
var maskQA = function(image) {
  return image.updateMask(image.select("SummaryQA").eq(0)).multiply(0.0001)
  .copyProperties(image, ['system:time_start'])
};

// Function for clip using the boundary
var clip = function(image){
  return image.clip(ncr)
}

// Adding the MODIS 16 days NDVI collection and select NDVI, EVI, and DetailedQA: maskQA
var mask_col = ee.ImageCollection("MODIS/061/MYD13Q1")
                 .filterDate(ee.Date('2003-01-