Thanks for the reply. The form is published to a list. All of the items appear in the list after submitting the form. However, occasionally, few items does not appear in the list. An email notification is sent to concerned person to take action when the form is submitted but the item disappears.
If the form has saved to the SharePoint list, it's not an InfoPath issue. The only way for them to disappear is for it to be deleted, or possibly the view has a filter that is hiding some items. Check the recycle bin and see if you can find them there. Also, set up an alert so that you are notified if anything is deleted going forward. Lastly, verify your view doesn't have any filters that might be the cause.