That almost certainly has nothing to do with you - hiring a fresh graduate through the H1B program is practically impossible in my experience. H1B applicants have to show "significant work experience" in a relevant field and unless you have a PhD (and often times even then), any work experience gained during your education will not count towards that requirement. Most companies won't even bother with the interview; 18 months sounds like the bare minimum to even have a chance at meeting the requirements.