Could you check what is your thread-pool-size in domain.xml? The default value of max-thread-pool-size is 200. If it is set to a low value, you might see only those number of active connections to postgres.
Alternatively, you could set the max-pool-size and steady-pool-size to the same value say 10 and print the physical connection toString in your app's code. If the physical connections output are all different at different times, then there is not a problem with the pool, else if the physical connections are all same or getting repeated, its an issue in the pool.
If this does not give you more info, could you post a test case (webapp) that you use to reproduce this problem? And also the OS that you are running this on.
Thanks,
Shalini.
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