I'm making a mini-game called The Scrollz. I made command blocks that give all players in the spawn room the equipment they chose in their loadouts. I tried it, and even when I do the "minecraft:" thing, it wont work unless I've powered it before.

How to reproduce:

Step 1: Create a /give command in a command block
Step 2: Power it with a repeater
Step 3: If it doesn't work, power it with another source
Step 4: Repeat Step 2

You'll see that you wont get the item until after you power it.