after reclassifying slope raster cells appear with no data

I have been trying to reclassify a slope raster, prior to reclassifying I have ran the statistics for the slope. Once reclassification is
complete and loaded cells that previously had data, have no data.
I have gone back and ran the focal statistics tool on the original slope and then tried reclassifying and there are still cells that previously had data, that now have no data.
Is there something that I am doing wrong?