Thanks for the response.
I managed to see where the problem was. The Data Type for the report field which could not be selected as a report filter select field was set to varchar(max) in the sql database. When this was changed to varchar(50), the field was available for report filtering.
I realized that I had the same problem with Crystal Reports 10 when I had already written to you. I created old reports with the data type set to varchar(50) long back which I later changed to varchar(max) and had not created new reports for this data table until today.
Many thanks for your help and effort.