If you are paying for 2 days shipping, you should get it in two days. It doesn't have to be complaining, but I would think they should be alerted to it every time they miss the deadline.
If this is happening to you quite often, then they should be digging in to find out where the breakdown is and trying to fix it.
Also, they give you the expected delivery date when you order, and sometimes it is more than the two business days, so you probably want to check that before making a complaint.
Personally, unless I need the product right away, I've been foregoing the 2 day shipping and racking up the $1 video/mp3 credits they've been handing out recently.