my last two beers have not carbed up as much as usual. one is an ipa with almost 8 percent abv . the other is a imperial stout. [almost 10 percent] I have been leaving them on the primary 3 to4 weeks then bottling. let them sit for another 4 weeks and they have little head that goes away too quickly. is the yeast dying off in the long primary? i used to bottle after ten days to two weeks and did knot have this problem
Nope...A month in primary is not enough time for the yeast to "die off." After 6 months, perhaps, but not a month (which is what most of us do these days anyway) there is still plenty of active yeast to do the job.
The fact that they are higher grav means that it will take longer than 4 weeks to carb up.
Those are big beers you have there, it will take time. The
3 weeks at 70 degrees, that that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Stouts and porters have taken me between 6 and 8 weeks to carb up..
I have a 1.090 Belgian strong that took three months to carb up.
Temp and gravity are the two factors that contribute to the time it takes to carb beer. But if a beer's not ready yet, or seems low carbed, and you added the right amount of sugar to it, then it's not stalled,
it's just not time yet.
Everything you need to know about carbing and conditioning, can be found here
Of Patience and Bottle Conditioning. With emphasis on the word,
"patience." 
If a beer isn't carbed by "x number of weeks" you just have to give them ore time. If you added your sugar, then the beer will carb up eventually, it's really a foolroof process. All beers will carb up eventually. A lot of new brewers think they have to "troubleshoot" a bottling issue, when there really is none, the beer knows how to carb itself. In fact if you run beersmiths carbing calculator, some lower grav beers don't even require additional sugar to reach their minimum level of carbonation. Just time.
Lazy Llama came up with a handy dandy chart to determine how long something takes in brewing, whether it's fermentation, carbonation, bottle conditioning....
Make sure they are above 70 degrees and give them some more time and they will be fine.
In the future if you do make higer grav beers like IIPAs and Barleywines, you could add some fresh dry yeast at bottling time...but for these just give them time.
