The higher the frequency of e-mails you send, the less they will be opened by members.
Regular, scheduled, e-mails with similar subject lines tend to be ignored.
This is why digests perform terribly compared with real-time notification of new activity.
We know what to expect in the digest. We believe we will browse it later, but we never do. It's never urgent. Never a priority. It never gets ready.
Even if you stumble upon a successful time, novel subject line style, and a format for an e-mail, that will only last until it becomes mundane. Then this too will be ignored.
Mix up the frequency, subject lines, and content of the e-mails which go out to members.
If open-rates decline, stop sending e-mails for a few weeks, then send two in one week. Use e-mails to bump up activity levels and notify members of major (genuinely) interesting things to pay attention to. Check the open rates and adapt as you go.