Getting back together after a break-up can be tough. It can be awkward, especially if one side has been mouthing off about the other, or if someone's gone and found a new beau, or if they keep saying "you can't be here Steven, this is why I have a restraining order".
Bands have it just as difficult. Money, brands, ownership, the custody battle of band assets can be a torrid affair, even if a band splits amicably. Egos run amok, and it can sour relationships forever. The bridges though can be mended, whether through the foresight of time, an event of great importance (see the plane crash that eventually brought Blink-182 back together), or money, artists usually find it within themselves to put their differences aside and reconcile for a tour and/or album.
Successes can vary, coming back after a break adds a lot of pressure to whatever output the band gives. If it's not up to the standards you all laid when you were together the first time around, what's the point in you coming back at all?
Some get it right and pick up where they left off or even eclipse their previous run. Others though are not so lucky and are permanently over-shadowed by their past selves.