Recipt notifications not getting for some purchage orders

Hi,

For some purchase order the Recipt notifications are not getting generated.
For expample PO number
30211102 
is approved but still it has not created any recipt notifiaction.
If i run the query

select * from applsys.wf_notifications where SUBJECT LIKE '%Confirm receipt of items%30211102 '  it does not rerurn any rows

whereas select * from applsys.wf_notifications where SUBJECT LIKE '%Confirm receipt of items% returns quite a few recent rows.
Most of the PO are sending notifiactions as expected but some of them are not.