The report which you are running in jasper server might be having more number of pages or fetching more data from the database and this is the reason the loading process is continuing and the export option is in disable state.
Unless all the pages of a report gets loaded into jasper server, you wont be able to export the report. So, you can apply following three approaches to solve this problem:
1. Limit the number of records in the query.
2. Apply filters to the query.
3. Optimize the query with more performance oriented.
In some cases, I've found that by navigating through the loaded pages will load all pages quickly in case the loading process is stuck.
I Hope these work around helps you.!
Regards,
Srikanth Kattam