In SAP ABAP, few records are skipped during parallel processing

Check out on code to handle parallel processing-

gv_semaphore = 0.
DESCRIBE TABLE lt_itab LINES lv_lines.
LOOP AT lt_itab INTO ls_itab.
CALL FUNCTION 'ZABC' STARTING NEW TASK taskname
DESTINATION IN GROUP srv_grp PERFORMING come_back ON END OF TASK
EXPORTING
...
EXCEPTIONS
...
.
" <<--
ENDLOOP.
WAIT UNTIL gv_semaphore = lv_lines.

Also, check the upper limit on a number of parallel process count. It may have some limit on a number of open threads.