Delta only allows you to hold an itinerary if you don't have enough miles in the account to ticket at that moment. (If you sweet talk the right agent, you might get a hold even if you have enough miles, but it's not easy.)

Here's a trick to avoid this annoying policy if you have enough miles in your account to ticket the award you want.

Call Delta at 800-323-2323.
Instead of giving the SkyMiles account number you'll be using, give the account number of a friend that has very few miles.