sost - still no entry in queue after commit work

Hi - We have a wrapper class(zcl_email) built around CL_BCS class and COMMIT WORK after send .

lv_sent_to_all = go_request->send( i_with_error_screen = 'X' ).
COMMIT WORK.

This email class(ZCL_EMAIL) is used WD application to send mails. In some cases we are getting "Still no entry in queue" error in SOST like one in a month.

Could you please help me why we are getting this issue. What could be the reason .