Under Primed, sweet tasting Heady Topper clone

I made a Heady Topper clone about 5 weeks ago, I've made it before and is my favorite IPA I have ever made.
It was AG, OG 1.084, FG 1.017.
I believe I bottled 52 beers with 3.9 oz. priming sugar.
Just like always I give them 2 full weeks and try 1, they are always fully carbonated.
They were in my basement, I am assuming it is about 68 deg. down there.
This time (after 2 weeks) I opened one and poured, it was very undercarbonated and extremely sweet. This has never happened, BUT my final gravity is usually 3 pts. higher!
I am assuming (Hoping) the sweetess is from the priming sugar.
I then moved all the beer into a slightly warmer room which is 72 degrees at a minimum.
Should I be concerned or is this what you would do.
Thanks for reading.

give it time

Did you add any yeast at bottling? With that high of an OG your yeast might have been very tired. Probably just got lucky in the past.

I did not add yeast @ bottling

Swirl each bottle and move to a warmed temp which you did. Try one again after a week you should see some progression in the warmer temp. But don't wait to long if they're not carbed after two weeks in the higher temp it may never happen in enough time before the hop starts to slip away. This just happened to me with a dipa I made that was 9.3%. Took forever to carb and by the time it did after I moved it to mid 70's temps the hops started to fade from the long carbonation lag time. I promised myself that day hat anything over 9% gets yeast at bottling. If u pitched a big enough stater your beer should carb with the warmer temps and some swirling of the bottles every. NOw and again. so..#,e

As mentioned earlier, adding some yeast to big beers will help the process along. I do this for higher gravity beers and sours before packaging. High alcohol (above ~6.5%) and low nutrient environment is inhospitable to yeasts after they ate through what they could in the primary. For sours, I add an aggressive saison yeast that will quickly clean up what the sour microbes take months to eat down.

If it's cloyingly sweet and not what you tasted on bottling day it's certainly because your yeast hasn't gotten going yet. You could make up a slurry of yeast and recap after dropping in measurable amounts. Oxidation shouldn't be worried about since a quick yeast will gobble up the available O2 in it's process. You can still recover your beer. Heady Topper clones would have a lot of oils and acids you want to preserve so getting oxygen out of the headspace sooner than later would be best.

I would take others' advice, warm it a bit more for a week and a bit, then chill it and try. If the yeast is dead, pop and add more (I use safale us-05 and sparkling wine yeasts since both are quick workers, alcohol tolerant, and don't impart notable flavors).