The last two times I have made Swiss Meringue Buttercream it has been really soft/thin.  It is not running but it soft and slightly thin.  I am using the the following receipe, except I heated the whites and sugar to 160F and used pasteurized eggs whites.  I was able to whip egg white and sugar mixture into a stiff Meringue. When I added the softened butter it never gets runny but the frosting is soft and thin.   Can anyone tell me why this happening or what I do to prevent it?

I take my eggs to 160 because they're fresh.  Since yours are already pasteurized  you only need to melt the sugar.  I don't know if double pasteurizing makes a difference or not.  My first thought is you're not getting the whites stiff enough, but you say they're stiff so I'm not sure.  Whenever I've used carton whites I have difficulty getting the whites really stiff.  My recipe would add 2 more egg whites and 1/2 cup more sugar, but I wouldn't think too much butter would make it runny.    You could try the 2 egg whites and more sugar.  I always get a pretty stiff buttercream so give it a try!

your meringue is not still too warm is it? it might tend to melt your butter and if the water separates out it could make it thin -- also you are using real butter yes? using margarine might work but a lot of the spreads out there won't work -- also your butter should not be too warm -- just soft enough to take the crisp off it -- 

in fact any butter type spread that is spreadabley soft when cold should be avoided 

just a couple more thoughts for you

Thank  you both for the suggestion.  I will try upping the eggs by two and adding half cup more sugar.   I use real unsalted butter.  I have been adding the butter to the Meringue when the bowl  when it is feels about room temperature when touched.  I try using butter that is softened as much.