Demetrius,
If you can reproduce the problem at will, then change the logging level for ExcelImporter to Trace to see any exceptions.
If you still cannot figure out what goes wrong, then import Excel into a staging table where each attribute is a string, and do the type conversion manually. With this approach you can execute significant amount of data validation before placing it into production table.
You can also abandon enumerations in favor of a lookup entity. I think Excel importer can populate lookup entities on the fly.