You need to monitor IMS OBA (amount allowed for overflow) and NBA (normal buffer allocation) by using IMS DBCTL with no IMS TRANSACTION MANAGER.
Resolving the problem
There are four region minors in classic (major command RGNx or PNRx) that display the NBA and OBA information. These are NBDF, NBIU, OBDF, and OBIU. For DBCTL, you need to use RGND or PNRx majors, not RGNF as this selects IFP regions that don't exist in DBCTL.
The EPILOG report, RFPR, and the TEP FPRGNS displays will not function in a DBCTL environment because you don't have IFP regions.
Also, the Thread displays in the TEP will show Fast Path buffer waits but not the actual allocations of the buffers.