My company sends a newsletter once a month. We're selling roasted coffee. This single day account for 20-30% of our monthly revenue. In fact email is our single best performing channel. Hope this info helps. EDIT: I should also add that we're combining every newsletter with some "offer" e.g. we make a new blend or roast couple of bags of some coffee we don't typically stock. We're also keeping it very personal - it's…
Sounds like you should switch to sending two a month.
This said, it's possible that once a month isn't the right interval. 3 weeks or 5 might be better. Analysis would need to be done.