The Muppets basically have the same problem they've had for the past couple decades - they do a major project then take too long deciding what the next major project should be that by the time it finally happens, they've "cooled" and it becomes another "comeback".
Even between The Muppets and MMW, even though the decision to make a sequel was done rather quickly, there was still do much time between the two and they weren't really doing anything between aside from dvd promotion. They went from the 2011 movie and major fullscale promotion to another major promo blitz when the dvd came out. Crickets. Promotional blitz for MMW where they had to get back into the public eye after having laid low for awhile (which is what i think affected the performance) then another promo blitz for MMW dvd.
But the thing is instead of doing a whole bunch of things around a release, they should be a fairly constant stream of little but effective things in between to keep their momentum going. Things like the Muppisodes (of which there should have been more) or just viral videos not connected with plugging the films should have been rolling out on the regular to keep up the interest. Keep things rolling like traveling on a highway regular and fast, not like a bus stopping then starting, stopping then starting. Keep them from "going anywhere" so they don't have to make a "comeback" when the next Biggie project is ready because they won't have gone anywhere.
I think part of that comes from the current organizational structure of The Muppets within Disney. At first there was a specific Muppets Studio division first headed up by Chris Curtin which was getting the Muppets off to a great start getting them out with multiple platforms and that was going well until internal politics across Disney leadership took him out and replaced with Russell Hampton who really wasn't a "Muppet person" and didn't really care too much about his job in that position. Thankfully that was shortlived and Lylle Breier took his place and it was more what you saw with Curtin's tenure but better (with some slight monkey wrenches thrown in the mix with the writers strike period). It was the various seeds Breier was planting that caused the Muppets' popularity to grow and lay the groundwork for the success they had with the 2011 movie. But around that time, from what i've been able to gather (i've never really gotten any clear answers on this), the operational structure of the Muppets, once they started working on the movie shifted to where Breier was no longer actually in charge of them and they've since basically been handled/managed by the movie promotion team. They need another Lylle Breier (if not herself back in the position) specifically looking after the Muppet franchise's best interests on a constant basis and not just patchwork streams of promotion once a movie or dvd is "ready". (And when doing the promotional blitz for the dvds, they need to remember that not everyone has blu-ray and that good extras need to be on both dvd and blu-ray - otherwise you're leaving out part of your target audience and wasting all this great new stuff on only a portion of those who will be able to view them thus creating a level of apathy in those who would normally be excited about the home release.)
Viral videos need to be a big part of that strategy again - we need the kind of stuff they were doing in the late 00's between projects as well as a return to what muppets.com was like at that time with regular updates and inspired random lunacy.