If a row in a source file contains non-numeric data for a field that is being used as an integer measure, that row is not included in the generated source table (and therefore isn't included in the cube either). The Import Data dialog gives no indication that any errors occurred.
The following source data from https://www.baseball-reference.com/teams/BOS/ reproduces this issue when the "GB" (games back) column is included as an integer measure - 19 rows are skipped because they contain "--" to represent 0 in that field:
redsox_seasons.csv