Thanks again, Ramanjaneyulu.
The solution was doing the Google search for 'Compression Problems related to Datamart Delta' and I came across this site: http://wiki.scn.sap.com/wiki/display/BI/Compression+Problems+related+to+Datamart+Delta. I executed program RSBK_LOGICAL_DELETE_DTP for the technical name of the DTP to the Open Hub, which is no longer needed, and was able to compress!
Mary Jo