SSRS inconsistent results from stored procedure

I have a stored procedure with startdate and enddate datetime parameters which returns 6 records when executed in SSMS like:exec sp_XXX '7/1/13', '7/31/13'.  If I paste this into the SSRS dataset as text it also renders the report with the 6 rows correctly.  When I change the dataset to stored procedure with the appropriate datetime parameters. when the report executes I get 13 rows non of which are in the correct result set.  I have displayed the parameters in the report and also returned them in the dataset and they look ok.