Usually MOM will not give reason for rejection. This practice of not giving reason for rejection is to prevent employers and prospective employees from artificially adjusting the application details in their favor and resubmit it again.
MOM will not reject you because of lack of documentation like SPM cert etc.. because to them, the lack of documentation is very trivial and if needed, MOM will ask you to show them the originals after they've given you an "in-principle approval".

Reason for rejection can either be:
1) Your salary or age does not meet the requirement for a TEP.
2) You graduated from an unrecognized university or your qualification is not good enough.
3) Company has reached its maximum quota for hiring foreigners.
4) The reason/excuse, the company has stated to MOM, for hiring you is not good enough.

Unless the employer has forgotten to mention that you're a first class honours student from Oxford during the initial application, an appeal will usually always end up rejected.

Most employers, especially mine, knows that appealing will ALWAYS end up reject so they usually make no effort to do it.

Time to move on, apply elsewhere.