Executing A Job From A Stored Procedure Stopped Working

I've got a stored procedure in database A that calls the sp_start_job stored procedure in msdb as follows:

CREATE PROCEDURE xxxxx
WITH EXECUTE AS 'domainusername'

AS

EXEC msdb.dbo.sp_start_job B'jobname' ;

RETURN

The domainusername is the in the database sysadmin role and the owner of the job. To make this work originally, I had to change the msdb database to be trusted.

This worked for the past several months.

Now it doesn't work (perhaps after a reboot but not sure). The error I get is "The EXECUTE permission was denied on the object 'sp_start_job', database 'msdb', schema 'dbo'

I looked to make sure that the account had grant execute rights and it does. I tried setting it via GRANT statement and it was granted successfully yet the error still occurs. I've tried changing accounts and anything else I can think of to no avail.

Any ideas how to troubleshoot this issue. I've tried all the tricks I can think of.

Thanks - SM