Work permit and re-entry

I have been offered a job in Singapore and recently made a trip there thinking I could speak to my potential employers face to face. But I was rejected entry into Singapore for unknown reasons. They refuse to let me know why and I had to board a Plane back to Korea.
I have since accepted the job and have gotten my approval on my work permit.
However, I worry if there will be any issues again with regards to my entry.
I want to avoid a repeat incident as the previous one has left an unpleasant taste.
Does anyone know what I should do to be sure it won't happen again?

Pray that you don't run into a German employee at customs.

Jokes aside, it's most likely due to your intent of entry. They may have thought that you wanted to enter Singapore for work and not as a visitor as your EP has not been issued.

Work Permits (WP) are not usually given to Koreans. Did you mean S-Pass or Employment Pass (EP) instead?
In any case, once you have your IPA, there should be no problem entering Singapore.

You didn't tell us why they denied your entry? Did you have valid visa, staying address in Singapore and return air ticket to enter Singapore? If yes, then I don't see any reason not to allow you Singapore? If you didn't have either of one or two or all, then you were rightly denied to enter here.

As Beppi said, once you have valid IPA letter issued by MoM, and offer letter then you should bring with these and show to immigration check point. Good luck