Plaese help to understand the reason why requestor couldn't edit own request via REST API?

http://xxx.xxx.xx.xx/sdpapi/request/137/?OPERATION_NAME=EDIT_REQUEST&TECHNICIAN_KEY=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&INPUT_DATA=<Operation><Details><parameter><name>subject</name><value>ТEST</value></parameter></Details></Operation>

.. and the answer is 

This request with engineers token works, but with requestors - not.

Thanks in advance.