Not all XY data displaying - but ArcMap does show it when Flashed...

Hi All,

I've used add XY data before, but not had this problem. Excel Lat Long data was put in an Access database, exported as a dB file (so to fix the data as double float) and then added in Arcmap. Display XY data was set, but only some of the points (albeit in the right locations) are shown. However, when the samples not shown are "Flashed", ArcMap flashes the right location, even though there is no dot. Exporting the data as a new shapefile doesn't solve the proble - it acts just as before.

Any ideas? All help very welcome!

Cheers,
Alex