Have had this issue a few times and not sure what's going on.
Using the Data Import Wizard fields just won't update. Most recently, wanted to update a custom field on Account. Uploaded a csv with Account ID and the field to update (numeric, custom field; pretty standard). Everything mapped. Uploaded with no error, yet the values for the field just did not change/update.
Have searched the forums, but still no clue what could be causing this as I can't find anyone else who's reporting this problem.
Used dataloader.io and it updated the field just fine, so no huge issue, just bugging me what could possibly be causing this.