Chemistry, as you say, is probably the #1 killer virus of collectives. The usual cause is disproportionate input. I'm working on an article about how groups cope that may shed some light.
My ongoing research into comics collectives frequently leads me to web pages announcing the birth of a new collective that I've never seen. Digging usually reveals that about half of these never get off the ground and the rest tank within about 18 months.
Friction between members -- chemistry -- is often cited, but what causes it? As best I can tell, unequal participation by members is the main problem. Some do all the work and others are along for the ride. Eventually the ones doing the work wonder why they are breaking a sweat and lose heart. Sometimes they receive an invitation to relocate and accept it, dooming the remaining group.
What works to overcome this problem and make a group successful? It turns out there are a variety of strategies, and I'm going to share them:
FRIENDSHIP - HalfPixel is a group of friends -- four guys who really get along and enjoy each other's company. Like the best collectives, they are united for mutual support and common goals, and have been quite successful at executing common projects. They regularly appear at conventions together, and recently published the How to Make Webcomics book, co-written by all four. They have a podcast and keep in regular touch. Launching a collaborative union would break many friendships, but these guys make it work.
STRONG WEBMASTER - Some groups are built around a site that is controlled by an individual. If that individual builds a good site, he can attract participants, and he can control strife by responding to member needs while gently reining in excess. He can also expel people without the formalities of a vote.
STRONG FOUNDER(S) - Few collectives have grown as quickly as TomGeeks, which went from seven members to 150 in a few months. As a collective for female comics artists, any woman with a decent comic is admitted. The founders have retained administrative status of the collective however, and that seems to suit most members fine. They are glad to be showcased on a well-designed site and to join the lively forum. Still, a group of seven in charge raises challenges and the specifics of their internal arrangements have not, to my knowledge, been publicly discussed.
COMMUNAL - Here, everyone has a say on everything, and it's hard to get anything done. Many groups that form without a governance plan default to this model before collapsing.
COMMON BANNER - Some groups post their member comics under a common flag, and usually creator bios as well. People are drawn to the group by its theme -- By Night, a horror comic collective, is an example. The cooperative is limited to the portal, and perhaps advertising it. Some members might never meet.
GROUP OF CASUAL FRIENDS - These usually start with the notion that forming a collective would be neat. Someone dashes off a logo, someone else throws together a site of sorts, then everyone adjourns to the forum for off-topic yakking. Usually a charismatic one is considered "in charge," but since there is no agenda, there isn't much to do except cajole someone to throw a post on the group blog once every six months.
Growth is a challenge to manage for every collective. Someone with a promising comic finds themselves feeling kind of odd and lonely while the others chat on about things. Having a sponsor helps -- someone to shepherd the newcomer along until they have been properly introduced and acclimated.
A collective can only manage a small proportion of quiet, seldom communicating types. A decision to carry a withdrawn person might be in the group's interest if they do a comic that will draw traffic, but fleshing out the ranks with people based solely on the quality of their work is dangerous to the energy level. Minor issues like keeping their section of the forum lively can become sore points.
First responders -- members who are always first to respond to any new post -- present the opposite problem. By depriving others of a chance to speak first, they present their point of view on everything until it starts to become the view of the collective to visitors. Enthusiasm is to be valued, but it can go too far.
To function, collectives, like any organization need methods to plan, control and execute. This is contrary to the freewheeling image of the collective many people hold, and some will object on ideological grounds. Good though their intentions may be, such people will bring ruin because granting concessions to ideology cripples performance.
Of all the organizing formats I have seen, one is curiously missing -- election of executive officers. It seems the choices are benevolent despot, friendship bonds, or anarchy.
Link for latecomers: The Directory of Webcomic Collectives