I brewed and kegged a Smoked Robust Porter, which is delicious, and have noticed the head was rich and thick at first, but now it is almost non existent although there is the same amount of carbonation. This is over the last two to three weeks after kegging, it was force carbed to about 2.25 saturation.
It is in my Keezer on tap which is set at 36*f while I lager a doppelbock. It is hooked up to co2 at 8-10 psi.
If I had to guess why I have no head retention on my porter it would be because I have it so cold but my head can’t get around that because I’ve had bottle conditioned beers in the past that only had head on them when they were near freezing…help me AHA forum… you’re my only hope :-\
Contaminated?
In theory, the temp has nothing to do with head retention. It could affect carbonation, though.
I’m going ot venture a guess that it has nothing to do with the beer but instead has something to do with your glasses. Did your wife change dishwasher detergent lately? Are you using a “spot free” rinse agent? Tyr hand washing a glass with super hot water and soap and make sure it is squeaky clean and hot rinsed and see if the problem persists.
This was my first thought as well, glasses that are not beer-clean.
So I upped the serving pressure on the keg waited a couple days more and got a clean glass and poured myself a glass of frosty Porter with a nice rocky head that was about 3/4" high and left lacy rings to the bottom of the glass. I must have been over reacting. I did what I forgot I learned from Charlie Papazian’s book, I relaxed didn’t worry and had a Home-brew and things were fine. Thank you to all who responded. I’ll try to come up with a better problem to stump you guys next time. 
Good deal