Two things can/will cause this, Marshall.
1) The torque converter is draining back into the pan, sometimes overnight, and sometimes it may take a couple of days.
The issue 99% of the time is the clearance between the converter hub, and the bushing in the pump. Too much clearance will allow air into the converter, and the fluid drains out. Then, it takes a minute or so to fill the converter with fluid after starting, which causes the delayed engagements. The only solution is to fix the problem, which is replacing the converter and/or the bushing.
There are also some valve body concerns that can cause converter drainback, but that usually only occurs after installing certain valve body kits.
2) The lip seals on the pistons in the trans may be hard, and not sealing well when cold. This is fairly common, especially on high mileage vehicles, and extremely prevalent on Chrysler vehicles. Only fix is rebuild.
Do this....next time, before starting it, check the fluid on the stick. Mark the level, or make a note of exactly where the fluid is. If the converter is draining, the fluid level will be extremely high, compared to a normal, cold level reading.
If the fluid level is about where it normally would be on a cold check, but the delay is still present, then it is a seal issue.
NO additive is gonna fix either of these problems. Sure, some of them will soften the seals a little and maybe make it better, but...there's nothing in the chemical composition of those additives that tells the seals when to
stop swelling, or getting soft. Eventually, the seals will simply fall apart, creating even more problems.
______________
Joel Adams
C3VR Lifetime Member #56 
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
). I gave it a moment and more gently pressed on the gas, at which point it started to roll. 
2014216132127r.jpg)
