OK, the problem is (I think), that the records are not being imported as there are already records in the Leads table. However these records are with flag "Deleted" set to 1. Any idea why import does not overwrite these deleted records?
Another idea would be to purge these records from Leads table. I did it through SQL, but worry there may be some records out of synch (garbage). Please advise
Import will only use the existing record if the ID is provided during import. This is rare, so I'm going to assume that it is not the source of the problem.
The most common problem with import is that the user does not realize the it is transactional and one error will cause the entire process to be aborted. In this case, you simply uncheck the Use Transaction flag and import again.