Newsletters are usually long-form content. They have lots of information. They take a lot of time to put together and get blasted out once a month.
You may see success with it by getting a spike in your business everytime you send it out. Does that mean you should send more newsletters?
Try this next month.
Take your newsletter and divide it up into four separate e-mails.
Segment your list into four groups.
Send each group a different e-mail for four weeks.
Now you will see a more consistent increase in sales instead of a spike.
You will also see which section of the newsletter created more engagement and sales.