Try implementing using custome code.

Seems that ur report has same group name for more than one grouping. Check for the names of all the groupings u have done.

If more than one group has...

Hope the following helps u out:

=IIF(Format(Fields!EnteredDate.Value,"MM") = Format(Now,"MM"), Cint(Fields!PC.Value), CInt(0))

Implement in the following way:

=Sum(<field_name.value>, "<group_name>")

<group_name> should be the name of the group and not the field name on which grouping is done.