First trace the problem. You can add these lines the snmp_rw.ora file in %ORACLE_HOME\network\admin:

dbsnmp.trace_level=16

dbsnmp.trace_directory={dir_name}

dbsnmp.trace_file={file name}

Try to start the agent. Check the trace file. If it has error like this:

nmiumini_initializeUM: Unable to initialize UQFailed while initializing user subsystem
Error initializing subsystems

Then you have the user initialization problem.