I've got a weird one here...
Importing from a csv file (UTF-8 encoded), with about 18,000 lines in it.
Created a dedicated import table, setup import script to Add New records (table is emptied prior to import each time).
The import runs fine, no errors, but only about 9,500 lines are imported. Can't really see any rhyme or reason as to why some lines aren't being imported - it seems to get not just the first 9,500 lines, indeed, the first and last lines are included, and many in-between, but just missing about 11,000 lines!
Have interrogated the source file to see if there's anything 'odd' about some of the missing lines, but they seem identical, in terms of form (not values) to all the other lines that come in fine.
There's no duplicate entries in the import lines.
There's no 'updating' going on here, just adding all records.