Zonal Statistics as a Table does not calculate statistics for all features in the dataset

I want to compute the mean elevation (slope, aspect) within an area of 800m around the nodes of a vector grid I have applied, using "Create Fishnet". Because my buffers overlap, I cannot use "Zonal Statistics". I tried to use "Zonal Statistics as Table" but the problem is that from the 197885 buffer zones, it calculates mean elevation only for 197340. Any idea why it would not calculate the statistics for the rest of the buffers?