Partial column formating

I have a workbook and one page is sorted by dates It stopped working and I couldn't see why so I selected the column and reformatted the dates to long date (January 04, 2014) so I could see what the problem was. Rows 4 - 38 changed as expected but 39 -105 remained as short date format (01/04/2014)
There must be something wrong with the data that excel can't change it to dates but I can't find it.