I have a report that has a date parameter. The default of that parameter is set using the expression "=Today()".
It works fine when executing the report ad hoc, but when creating a subscription it causes a problem. Normally, there is a radio button to tell the subscription to use the report default value. For this partcular parameter, the readio button is not available. The subscription will set the parameter value upon creation of the subscription, but it stays that value for each subsequent run instead of re-evaluating the expression for each successive execution.
The problem seems to be isolated to when the parameter is set by expression.
I was able to duplicate the issue on other parameters by changing them to an expression and the radio button disappeared for them, as well. I can fix the problem with the date by using a query-based default, but it doesn't really fix the problem.