Kaplan's diploma IMO is not recognised, it is a mere fast track solution for those who wants to further their undergraduate studies under Distance Learning. It is of no contribution value. I just want to understand why you didn't choose to pursue local polytechnic after your ITE?
Secondly, your ITE certification in Comp (engineering) is totally unrelated to your diploma studies/qualifications. So the diploma is basically a mere diploma qualification and adds no further value to the field of sociology or counselling.
I now got 1 simple solution, get an advance diploma from local polytechnics studying on a part time basis. From what I know RP and SP offers them. The advance diploma takes less then a year.
Otherwise, you can consider heeding Unisim's advice of taking SAT and/or retake O levels. My take on why that advice is because they want to gauge your standard.
So before I end this lengthy speech of mine. Take a look at the criteria of Unisim's admission.
•GCE 'A' level with two passes (prior to 2006) or two H2 passes (from 2006), or local Polytechnic Diploma or International Baccalaureate (IB) Diploma or NUS High School Diploma
•Two years of work experience or is currently employed on a full-time basis
•21 years old and above
So you see why you didn't got accepted? Private diploma are subjected to case by case basis and as I explained, its not really recognised unless for Distance Learning degree purposes.
Unless you have strong related work experience, you may get accepted and that you have to gauge for yourself.
You may also considered getting referees from your work place if necessary to strengthen your application as stated "passion in the field"