Hello Georgina!
Is there any chance you could share the Excel file?
There are a number of factors that come into play when importing data from external sources, especially Excel files. Sometimes CSVs are a bit more cooperative (as long as you don't have data that has commas, such as a text field or single-field address entries that have commas between street, city, and state). But taking a look at the data would help us do some testing to find out why you're seeing what you're seeing.
Just off the top of my head, there could be invalid characters in the headers, spaces somewhere (leading or trailing, both in headers and in the data), or something similar triggering the issue. If you import the data as just a table (let's say, into a file geodatabase), do you see the same data you did in Excel or does anything change? Sometimes seeing this intermediary table helps recognize problems.
If you need to, you can give some falsified data, as long as it reproduces the issue.
Keep us posted!
Best regards,
Rachel
Esri Support Services
Rachel Guttmacher
ArcGIS Online Technology Lead
Esri Support Services