If you are using XLS files (not XLSX) and using the JET OLE db connection string, in RegEdit, navigate to this registry key
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Jet\4.0\Engines\Excel]
and set the TypeGuessRows field to 0.
If this doesn't solve the issue, you'll need to share the XLS file with us to investigate.
#XPression