REP-0730 error binding a formula function query to group item

 I am developing a report with a simple query and several formula items. The queries for these formula items need to bind to one or more group items (from the report query). Am encountering intermittent problems with "bad bind variable" errors.

1. E.g., the main query is: SELECT ITEM_NO, ITEM_ID, WHSE_CODE from...

2. I add formula items to the group with select statements for various history data for certain periods, e.g. SELECT SUM(TRANS_QTY) FROM IC_TRAN_PND WHERE ITEM_ID=:ITEM_ID AND WHSE_CODE=:WHSE_CODE AND...

3. Several of these compile fine, but then one doesn't, I get the above error for the bind variables. I thought I had "fixed" one instance of this by re-ordering the group (by renaming formula items) so that the group item to be bound (e.g. ITEM_ID) came before the formula item referencing it (in the data model).