I have 300 hrs, mostly SEP and about 40 hrs MEP, MCC and JOC and a university degree
That might be the problem - although other candidates might have similar hours, they probably attended an 'approved course' rather than hours building as your experience seems to suggest?
maybe for NAS 300 hrs for my age is way too low
That is low and your age/experience profile does not fit the 'age commensurate with experience' criteria (of other airlines, NAS doesn't quote this).
I know pilots who have gained employment with similar levels of experience to yours, but not straight on to jets and/or without attending an approved course.