Most of us have to be careful what we say on Twitter.
It's (usually) public. Lots of people read it. It gets indexed. It lasts forever.
One bad/foolish comment can destroy a reputation, even a career.
People are more worried about looking bad than looking good.
The bigger the audience, the less anonymity they have, the more afraid they are.
Your members are terrified of asking dumb questions, being criticised, providing information which is corrected by others, being outsmarted by perceived rivals, unintentionally upsetting someone etc…
This isn't the case with WhatsApp/Snapchat. You can say what you want to a tiny group of people. You worry less about the identity you've spend a long time constructing. You can be more provocative, more yourself. Fear stops more people contributing than we realize.
Most of us know this of course, yet few do anything about it.
There are three core issues here you can influence:
1) Size of the group which sees the message. This is the biggest one. We can say more when we know and trust the small group of people that see it (usually less than 10). If you allow people to create their own, small, simple, private groups from within the community, activity might rise.
2) Longevity of the contributions. If a message lives forever, we have to be worried about it. You can consider not indexing contributing and allowing old discussions to disappear (like 4Chan).
3) Non-identifiable contributions. You can give members a button to submit comments anonymously, use any nickname they like, use throwaway accounts (Reddit), or otherwise mask who the contribution is from.
We've found this more true in environments where identity is important (profession), than consumer-led communities. We haven't scratched the surface of privacy and anonymity yet. We need to explore group size, contribution longevity, and anonymity to encourage more honest contributions to a community.