Hi Vic,
Neither your salary nor your education can be the problem, since you are above the highest threshold of S$8000/month.
So the problem must lie with your company, or your job.
Your company must fulfil certain criteria to be allowed to employ foreigners (e.g. there are quotas and other considerations). Ask ACRA or MoM if these are fulfilled in your case.
Your job must be impossible to fill with locals. Prove that to MoM (e.g. by showing them the poor results of the online ad you mentioned above).
Or, to avoid any rejection based on your company, apply for a PEP instead. That requires a previous salary (i.e. what you earn currently abroad) of above S$8000/month.
Of course there are other possible reasons for rejection, e.g. if you are an ex-Singapore-citizen, left Singapore on bad standingin the past, or have a criminal conviction (anywhere in the world). Or your applications contained wrong or contradicting information.
In any case, MoM's decision is final and there is nothing you can do if you exhausted the possibilities to appeal and re-apply.
Hope this helps.
Beppi