Purchase Requisition workflow :WS00000038 not triggering in Case of Cancel event

Dear Experts,

We are looking for Purchase Requisition workflow .

We are copied standard workflow WS00000038 and created new workflow and done binding .

We have assigned agent properly.

Our workflow works in case of Release and Reject of PR but in case of cancellation of Purchase requisition workflow not triggering .

We need to intimate the creator of PR that "PR Cancelled".

Guide us on the same.

Thanks

Surekha