Hi Jayanta,
The spreadsheet datatype was 'General' and this has always worked previously when importing this data for XY display. I don't see why this shouldn't still work, unless ArcMAP has changed the way it handles spreadsheet (Excel) datatypes. There have been absolutely no conscious changes to my workflow in this aspect of my work.
Anyway, I went back to the spreadsheet, changed all the relevant cells to 'Number', saved it to a new name, then tried to display the XY data, but nothing changed. So that didn't work. When I looked at the table in ArcMAP the Field data type said 'Text' - it had apparently changed it (read/converted it?) from 'General' datatype in Excel as 'Text': that I don't understand.
I find it interesting that my first imports of Excel files on this project worked just fine, but seemingly gradually began to fail. The first loss was jus the two fields Dip S0 and Dip Dir S0, but I could see the others ok (Excel cell datatype for those was also 'General'). I actually recompiled the data in a new spreadsheet from scratch (~350 GPS stations with multiple data entry per station), displayed it, but still saw none of the fields.
The good news is, I resorted to your second option and restructured the imported table (where the fields were actually still present) and changed the cell type to 'Integer'. This did work, thank goodness. This seems to me to add support to my view that ArcMAP 10.4.1 is handling this type of data differently somehow. I then checked the Datatype against fields for the layer and now, instead of Integer, it reads 'Long'...! Sorry, but I don't understand what's happening, or why.
So, your reply was helpful, but only part. Changing the data type in cell to Number had no effect and the problem persisted.
Thanks for your reply. Appreciated.