When loading custom fields, that are drop downs, multiple choice or any others which already have a value in them, you need to make sure that what you are loading in MATCHES what is already there IE
If in your spreadsheet you have a column for "Season" and the data is "2015/16 Season" then that is what needs to be in the system. If the custom field which you are loading the data into simply has "2015 - 2016" then the values are different and the system won't know to match them
Spreadsheet | Custom Field | Loaded? |
2015/2016 Season | 2015 - 2016 | NO |
2015 - 2016 | 2015/2016 Season | NO |
2015 - 2016 | 2015 - 2016 | YES |