Why Some Results Missed from UNION?

[唐亮]

Hi dear pigs,
I got a problem:

When I use UNION command to combine some results in one relation at the end
of pig script,
it sometimes will miss some results from UNION.

For example:
union_all_res = *union* res1, res2, res3, res4;
dump union_all_res;  -- Or store union_all_res into '...'

It will sometimes miss some results(such as res1, res2, res3, res4).
But sometimes DUMP will show the complete results.

Please help me to figure out the problem.
Thanks!!