First, I'll assume that you did your ARRT certifying exam in radiography, because if not, there's the most significant problem right there.
Otherwise, there's no way to be 100% sure about what the problem is; it could be one, some, or all of the fact that you're a new grad, you can only do weekend hours, you haven't applied to enough places, you aren't selling yourself well enough, or something related to the tight economy.
As for contacting the person in charge of the department, it may help, and even if it doesn't help, it won't hurt. (I would recommend doing this by appointment, rather than showing up unannounced.) It is a chance to sell yourself to a potential employer, and at the very least, a way to practice interview skills. But keep in mind that hospitals usually handle hiring through HR departments (though the relevant department head makes the actual hiring decision), which have applicant tracking systems (ATS) that store applicants' files. So if you have already sent in an application to a hospital, it should be on file and accessible to the manager if s/he does a search.