problem with notifications module in drupal

i have installed notifications module. It was running fine but some how the records in "nofitications_table" are deleted

i don't know where the records are gone and how they are开发者_StackOverflow deleted, because of that the the mail is not delivered to members

i looked in to the code i didn't get any idea how the records are deleted

is records are deleted by cron ?

can somebody help me what is going on with this module