Also just a guess but maybe your excel cell formatting has an issue? maybe there are different types in the same column? the exporter does not like that.
A second guess is date time formats! If your excel has date entries, make sure that your import template pays respect to the pattern :)
Just an Idea: Did you set the importer to synchronize existing objects? If so, your key may not be unique what may cause the importer to overwrite the same record over and over again.