It can be frustrating to only receive part of your order, but it is common for orders containing multiple packages to have them delivered on different days, even though they were shipped at the same time.  Here's exactly why this happens (click Play on video below):

As you can see, it is very easy for parcels to become separated from their mates.  This means they will arrive on different days.  If you have ordered multiple items, simply wait and the rest should arrive within 72 hours  :)