1) Good stuff, they like engineering pple.
2) The fact that they called you up before, shows that they were satisfied with your academic qualifications. Many guys don't make it the first time either, so don't worry too much about it. Think about the possible reasons why they didn't let u thru to the 2nd round tho.
3) They process a lot of applications, so it's possible that they've overlooked yours. Nothing much you can do except to keep trying, unless you know someone who can help highlight your application to the HR pple e.g. captains, other SIA staff.
4) SIA is still taking in diploma holders, as long as you've fulfilled the basic pre-requisites (O lvls).
5)
AFAIK, there's no limit to the number of times one can go for the 1st interview. Heard that the time between the 1st interview and next application has to be 6 months, altho I can't confirm that. So you WILL be given a 2nd chance, unless for some reason you've been blackmarked.
6) Go for it! Keep applying. In the meantime, get yourself a degree. The higher your education, the higher your chances.
Btw, how old are you?