Originally Posted by Bowgie

It can take a while for Delta to recognize and fix computer problems.
Specific to your problem:

1. You're a Plat. I thought Plats got instant upgrades to DC+ immediately after booking. I just booked about a dozen segments for my Plat wife. Every one of them allowed me to check "Request Upgrade to DC+ (and to First Class with another check-box). Immediately afterwards, which is just after booking, I can change her seats to DC+

2. For those First Class non-upgrades, have you noticed if the First Class cabin's seats are getting filled? This is hardly definitive, but might give an indication of whether you're actually getting skipped, or ---

3. Perhaps the upgrades didn't actually clear and Delta is sending out a bunch of "false positive" e-mails saying it did. If this is the case, I'd be a whole lot less upset than actually being bypassed in the upgrade queue.