Platinum God not unlocking!

Question

I just beat the last Rebirth challenge, #7 Suicide King, and got the Suicide King Achievement. I waited. And waited... No Platinum God?
I looked back at my Steam achievements to see that I had all of them up to Platinum God, and others beyond (Dark Baby, Blinding Baby, Eden's blessing, etc.). Puzzled, I went back to the game stats, and under items, I was missing like 15 of them on the first three pages. Why?
I had unlocked all of their achievements, even picked up all of the missing items in runs (Missing No., Razor Blade, Forget-Me-Now, Epic Fetus just to name a few), but they didn't show up in my items page!

Nope, no seeds. And I've gotten all of them I'm pretty sure. Also, to check, one of the missing items was Guardian Angel (Which I've gotten) and so I started a Maggie run. I beat Satan and The Lamb (Thanks to Death's Touch, Continuum, Guppy) and no achievements appeared or anything. I then went to my items list again to find that it's still not there!

Answer 1

If you miss those items but you are sure you've found them, probably you picked them up while playing a Challenge, or a Daily Run. They work the same as a seeded run, so the game doesn't record any item or achievement.

Answer 2

Platinum God (as well as Real Platinum God) is not unlocked simply by unlocking all secrets. You have to encounter (and pick up) all items at least once. The full requirements can be found at: http://bindingofisaacrebirth.gamepedia.com/Achievements

Collect all items in the game, unlock all of the secrets and Endings minus The Lost and his 7 unlockable items

As is for all achievements, progress is not counted during seeded runs or challenge runs. If there are question marks on your Collection Page, those are the items you have yet to encounter.

To see what items correspond to each question mark, look at the chart here: http://bindingofisaacrebirth.gamepedia.com/Collection_Page