Unexplainable Exclusive Transaction Locks?

I have a job which runs the procedure below concurrently in 4 different sessions; It updates two tables.

PROCEDURE UPDATE_TAB_1_AND_TAB_2( traceIdTab IN pl_sql table,
individualIdTab IN pl_sql table,
fileSeqIdTab IN pl_sql table,

[Code].....

Each session is passed a sequence generated file ID; so the records processed in each session are completely different.

However, everytime this job runs, without failure, ONLY TWO sessions process concurrently, while the other two sessions are blocked. Once the first two have finished, then the remaining two sessions start work.

I have being monitoring the sessions on each run daily, and realized that the first two sessions processing are each holding a transaction exclusive lock on objects; hence blocking the other two sessions.

When I tried to find the objects that the two running sessions have a lock on, I cant seem to find anything by searching DBA_OBJECTS.object_id. However, when I look for the objects that are being waited on, its either a partitioned table or a partitioned index on either table_1 or table_2.

At first, I thought it was an ITL lock and after much analysis, I did not find anything which definitely pointed to an ITL lock. However, just to make sure, I increased the INITTRANS value from 2 to 5, but the problem did not go away.

At this point, I have applied all I know to understand the origin of the TX lock and how I can work around this situation, and decided to ask higher powers in Oracle for advise.