Sorry to hear about this, usually AMEX is consistent with enabling CLI.
There are several points to clarify first, however, just to be certain you aren't hindering yourself with your actions.
1) Which sequence did the BCP and Delta card (original) get applied? Did you get the $4k first, or the $1k first?
2) Have you gotten any CLI on the BCP to get it to $4k, or it has always been at $4k? If any CLI were approved, what day was that?
3) When was your last request for a CLI?
4) What is your spend / usage volume on each card, including the Platinum?
Now, recapping some of the rules:
1) Once you get a CLI on an AMEX Revolver, you have to wait 6 months before asking for any CLI on any other AMEX revolver.
2) Next, if you ask and are denied a CLI, then you must wait at least 90 days before requesting again.
If you move too fast, and get these two blocks working against you, you can perpetually try and fail to get a CLI because not enough time has passed.
Said a different way, the AMEX CLI is not a Luv Button like some other banks where you just keep pressing it until something happens. With AMEX you really have to plan and wait for the time limits to pass be fore doing anything.
What is you credit score on EX?
And for what it is worth, NFCU credit limits are like outer space alien limits; they won't translate to regular bank credit limits 
Ok, well it may have something to do with income level and having 3 cards with AMEX. With the NPSL and two revolvers, they might feel that is sufficient for you for now. If you were asking too often, that might also be a factor. Waiting 4 months now would be advisable.
I'd suggest continuing to use and pay heavily, right up to the $1k limit. My Delta Gold was at $2k when they issued it to me in mid-2013, and was really my first Centurion Bank AMEX card. I used it regularly for $500 - $600 per month, then in June 2014 started really running larger charges through it. Got it up within about $50 of the $2k limit, paid it down, and the next time it got up near $2k, they gave me an auto CLI to $3k.
The NPSL Platinum could be your often-used card. The MR points you earn there are of course transferrable to Delta SkyMiles, so using that card is a more free-form way to get higher credit availability, for PIF purchases of course, and run even more charges through the set of cards.
The other option to consider, if the BCP is more than enough, you can transfer limits between cards if the donor card has been open 13 statement or more.
That is still strange, however, that the Delta Reserve card, with a $450 AF, has only a $1k limit. I thought there was some rule in the CARD act about requiring CC limits to be some multiple of any AF, and it was certainly more than 2x. This was to keep predators such as CreditOne and First Premier from giving a $200 CL and charging a $200 AF, which is almost where your Delta Reserve card is.
