When IDMS tries to store a record on the database, IDMS will try to store the entire record on the first page.
If it can not store the complete record on the first page it will try to store the record on the next page. IDMS will go through the entire database area looking for a page that still has enough space to store the entire record. If there is not enough space on any one page to store the complete record, the DC900432 Error Status = 1211 message will be issued.
When you run DSEXSTAT it is looking at the entire database area and calculating how much space is cumulatively being used within that area. DSEXSTAT is not looking at individual PAGE utilization.
So even though the DSEXSTAT may show that 5% of a database area is not being utilized, there is still not enough space on any specific page to fit the entire record that needs to be stored.
The same is true for INDEX areas of the IDMS database as well.
To correct the problem you either need to delete some of the records from the database area to free up space on the individual pages within an area. Or, Increase the size of the affected database file/area using the MIGRATION process documented in the Systems Programmer guide in chapter 6.