Hi,
No one is doubting that... you may have passed your 12th exam honestly , BUT... low percentage is low percentage.
Marks of the last major degree (in your case 12th), relevant course selection, good IELTS score, proper finance and overall consistent profile is what V.Os are usually looking for.
Your low percentage in 12th seems to be the only problem. Even though 50% is the minimum requirement, each case differs. Some get visa with 55% and some don't even with 65%. So nothing is sure. In your case..you have taken a sharp dive in performance.. from distinction in 10th to a second class in 12th... this shows (in the eyes of V.O) degradation and no consistency. If this was other way round (second class in 10th and distinction in 12th..then it wd have clearly shown progress). Although, this is just an assumption... and unless you see the CIAPS notes..you will never know the exact reason.
Try to improve the IELTS score and write a very good SOP. Best will be to take admission in a B.E(4 years degree) COURSE IN some public UNIVERSITY, AND THEN APPLY FOR VISA ON IT (provided you can get admission in B.E). Your chances will be much higher, once you go there..change over to diploma.
Best of Luck.
*** All opinions expressed are purely personal, and pieces of information mentioned are to the best of my knowledge... You should consult other sources too.