One thing I would say that may have nothing to do with your problem is there isn't a need to execute your proc using the execute immediate syntax. You could just hang it off your transaction object and call it directly like any other proc passing it your argument. I only use the execute immediate syntax when building the sql dynamically at runtime.
You say it works on the backend when you call the proc one right after another. I would try the same thing in PB by eliminating the loop there...
SQLCA.of_execute("Exec calc 3")
SQLCA.of_execute("Exec calc 6")
SQLCA.of_execute("Exec calc 9")
SQLCA.of_execute("Exec calc 12")
You may want to check where you're performing a commit.
hth,
Mark