Hard to answer without details about a specific project; and I guarantee you that everyone involved will have a different point of view!  That said, here are some theories: it could be that there were problems in the walls that before opening were undetectable.  Depending on the extent of electrical, plumbing, mold or other issues, this could significantly extend a project but is in no way anyone’s fault.  Stuff happens.  I would say ditto for Asbestos and certain types of wall insulation.  It could be that someone involved with the project became ill or was terminated for some reason.  Once or twice I’ve heard of projects going through several designers or contractors in succession (it never seems to be architects).  Choose wisely.  Specific building requirements are often the culprit, although generally buildings in NYC want you in and out in a specified number of days (this may not apply where you live).  I did have one client who had problems with her wood plank buckling.  The building allowed a removal and reinstall.  Then there was a flood & the process needed to be repeated.  The building’s freight elevator was booked for a year ahead and, at that point, the client had to wait their turn.  Sadly they were unable to move in until the work was resolved.  Materials NOT being on site at the start of construction and clients who make changes to the design plan after construction begins are also culprits.  In NYC you also run the risk of leaks and inadvertent damage to surrounding apartments.

If we’re talking about furniture, order from overseas at your own risk.  This can often mean 18-20 weeks if there are delays.  More locally, manufacturers go out of business and discontinue items all the time without notice; and in today’s economy anything that is not stocked, i.e. custom orders, may not even be submitted until the vendor has enough orders to satisfy a manufacturer minimum!  Also, recently I’ve noticed that some of the Big Box stores are not even warehousing merchandise locally and it will take several days to several weeks to receive your order.  In addition, floor samples seem no longer to be for sale, even when they’re the last one!