Kriging creates holes in resulting raster

Hi! I'm very new to spatial analysis, and I was wondering how to fix this problem. I have a set of points that convert very nicely into a raster with kriging, but a portion of the raster is missing. Why does this happen and how can I fix it?
I've put an example below. It probably has something to do with the large number of points in close proximity there.
Thanks!
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