What does it mean when Reserve promotions are delayed?

Well my friend SSG(P) (Join to see) promotion requires both authorization [successfully meeting the requirements to be promoted] and appropriation of sufficient funds to cover the pay increase.
A significant factor is end-strength ceilings. In other words sufficient number of NCOs need to retire or separate to "make room" for other soldiers to be promoted into or within the NCO ranks. In times of high retention that can delay promotions.
The key factor in a promotion is the effective date which can be before you are actually promoted [but never after you are promoted.]
You may well be entitled to back pay if the effective date is months before your actual promotion.
Any corrections to my post? SSG(P) James J. Palmer IV aka "JP4" SGT Mark Anderson SGT Jim ArnoldSFC (Join to see) SGT Steve McFarland MSG Andrew White MSG (Join to see) SFC Joe S. Davis Jr., MSM, DSL SGT Ben Keen PFC (Join to see) SPC Elijah J. Henry, MBA SSG (Join to see) COL Mikel J. Burroughs

There's an issue with the slating system and orders are not being published efficiently. When they are published, they are supposed to be backdated but that may be wishful thinking.
If they do backdate, you'll receive all the back-pay but that's only if you should have already been promoted.

You should check with your ARA or S-1 and ensure your PPRL entry has your correct mileage and points.
If there isn't a open position with your MOS and your next grade, within your radius, you won't be promoted.
If you do have a decent radius, 100-300 miles, it should just be a matter of patience.