I don't think there is a problem with calling back and checking again. When you do, just remind them politely what they told you the last time you called, and that you are just following their guidelines. If they still do not have an answer, be sure and get another, new guideline from them and then check back again if that deadline passes.
Having been on the hiring end of things a couple hundred times, it never bothered me if a candidate called back to check on things as long as they were not pestering me every day. If I was telling them that matters would take 2 weeks, 3 weeks, whatever to decided, and that they would hear from us.......I would expect someone to call back when that time had passed.
Lots of things can happen. Budgets change. Schedules change. Some candidates they really want to at least interview cannot interview as soon as they expected. Unless you are choosing between this and another offer, be patient, and keep in contact with them. Don't wait forever, but if it seems like what they are telling you may be legit, give it a little more time.