Occasional 4x4 delay

I've noticed that every once in awhile there is a delay when going from 2hi to 4hi. The 4x4 green indicator light will flash for maybe 5 -7 seconds or so before engaging. Once when it happened I simply let off the gas and it engaged and the other time I was just coasting and I gave it a little gas and it engaged. All times were less than 40 mph. I know the manual states if it continues to flash then operate the switch back to 2hi and try again (and I've done that and it always worked). I'm not thinking anything is wrong, but I'm mostly curious. Anyone know what can cause it to have the occasional delay? 95% of the time it's a very quick change over though, and going back to 2hi is almost instant. Thanks in advance.

 

With the MT this is a common thing. Letting off the gas, shifting to another gear, giving it more gas etc etc makes the light go solid every time. The 4WD in the truck is already engaged in 99% of these scenarios where the light is still flashing green.