Problem:

It may happen that an ISAM file is corrupted and needs to be rebuilt, but when the rebuild utility runs not all records are returned.

Resolution:

This could be caused by a physical corruption  in the data (.dat) portion of the file.

A "rebuild MyFile.dat" only builds a new IDX file and keeps the DAT file unchanged.

Use the rebuild with InFile and OutFile and the option /d to rebuild a corrupted data file:

REBUILD.EXE Infile,Outfile /d