Initially was ASM, for 500.
I collected the passbook just now (they called me saying the issue was fixed), i asked to topup ASW, managed to grab 500 too.
Had a pretty clear view on how they performed the transaction, basically the whole thing takes less than 1 minute.
From the moment they login to the system (here took more time to load the page), fill up our details (only have to key in our account number & amount), the rest like which fund, payment type, initial/additional subscription are all drop-down selection. Then click on save button, confirmation box prompts out, then click confirm button and it will show you whether it is successful or insufficient fund (took only few seconds).
So, like what other ppl mentioned before, it is purely on luck (whether the fund is available that time) and if the staff willing to help you.

finally they managed to fixed the problem.
now onwards hope can do ASNB transaction without any problem at the branch.