Stored Procedure Denies EXECUTE Permission

Suddenly a stored procedure, very much like several others, is giving
EXECUTE permission denied on object 'Add_Adjustment', database
'InStab', owner 'zhoskin'.
server:Msg 229, Level 14, State 5, Procedure Add_Adjustment, Line 18.

I'm zhoskin. I am the dbo and created the procedure, and when I look
at its properties, I have EXEC permission. Line 18 is just the return
statement. The values are all appropriate for the table. So what is
usually going on when a stored procedure denies access to its owner?

Thanks//Zeke Hoskin


CREATE Procedure Add_Adjustment (@AdjAcc Int, @AdjType Char, @AdjAmt
Money, @AdjYrMth Int, @AdjDate Datetime)
/* Add a Dep Adj (Type Z, Negative) or WD Adj (type Y, Positive) */
AS
IF @AdjType = 'Z'
BEGIN
Insert Into tblTxn(TxnAcc, TxnType, TxnAmt, TxnSign, TxnYrMth,
TxnDate)
VALUES(@AdjAcc, @AdjType, @AdjAmt, -1, @AdjYrMth, @AdjDate)
END

IF @AdjType = 'Y'
BEGIN
Insert Into tblTxn(TxnAcc, TxnType, TxnAmt, TxnSign, TxnYrMth,
TxnDate)
VALUES(@AdjAcc, @AdjType, @AdjAmt, 1, @AdjYrMth, @AdjDate)
/*this is just to afect line numbers*/
END
/* set nocount on */
/*space holder*/
return
/*more space*/
GO