There are several reasons why a series might be underperforming:
1. The series does not appeal to a large audience.
2. The quality of the writing gets in the way of sales. No sell through is a big tell in this case.
3. Improper marketing -- categories, keywords, covers, blurbs, leading to a lack of visibility.
4. Lag time between launches.
In case #1, there's really not much you can do. Even if you reached every potential reader in your target market, you might not make much money on that cross genre non-tropey category-bending story.
In case #2, you can get an edit, either line or developmental, which might help you see the story's flaws and fix them. I remember early days in the series I posed above when I got beta readers who offered suggestions and when I made the edits, wow. The result was significant. A good edit can make a story flow better, get rid of writerisms, and help with pace and other issues. I find issues with pace are key. A great opening and a great pace (appropriate to your genre) keeps readers turning the pages and wanting to buy the next book in the series.
In case #3, the right categories and keywords can mean that your book is put in front of the right potential readers. The covers and blurbs may make your book irresistible to those readers once they see it. If your potential readers never see your book because it's mis-categorized or has bad covers or an unappealing blurb, they may never click on that purchase button. You can fix this by recategorizing, revising blurbs, and getting genre-appropriate covers. You can also help by advertising to the right readers, using comparable authors and books as interests in whatever platform you choose to advertise. That's a good way to find readers who like your kind of book.
In case #4, those readers who are impatient to read the next book in a series may forget your name when they move on to someone else. Unless you are a huge seller, this is quite possible. I had someone who bought my first big seller back in 2013 who only learned three years later that Book 2 was out. It happens. The longer you go between launches, the greater the chance that your first readers will forget about you.
I think you can definitely relaunch a series that is faltering or that never took off. The issue is whether you have a product that has a market and whether you have been unsuccessful in getting the book in front of that market. Once you do, it's possible to find success.
My series is a case in point. The first three books sold pretty poorly in the first years. The original covers were not appropriate to the market. The first books were not edited beyond a proofread and so there were a number of places where the pace and prose could be improved. Nothing much happened until I got new covers and went wide with a permafree first book. The series started to sell. I also started to advertise the boxed set and that's when things definitely improved. Every book sold better. I released the next book and the series made mid-five figures that year and the next year.
I only spent a lot of money on advertising once the series was showing a positive ROI. I made enough money and more so that advertising was well-worth the price. In other words, I put in minimum investment in advertising and relaunching up front until I saw results and then I doubled down. I didn't go all out before the relaunch and spend thousands of dollars. I spent about $550 total on new covers and nothing to go wide with a permafree first in series. Once sales started to increase, I considered advertising and took Mark Dawson's Facebook Ads course. I started out with $5 a day ads and when that showed results, I upped spend to probably $50 a day and had 300 - 400% ROI for months. Facebook ads kicked my series into high gear. THEN and only then did I finally get a Bookbub, which meant my books sold even more with a nice long tail.
This is the chart from Book Report for the series in 2015. You can see I was in KU 1.0 in January. I pulled my books out in February and put them all wide by March 2015. I started to advertise in April and you can see the step up in revenue in April and May and then a slight decrease in June. In July, I had my very first Bookbub for the series -- a free promo for the first in series. You can see it was a very good month. The revenues dropped back down in August and I pulled it off wide distribution in September and put it in KU 2.0 but you can see the 30-day cliffs clearly like a stair case going down. I pulled it back wide in December and ended the year back at a pretty low level because of my stint in KU. Of course, this is a graph just of Amazon and doesn't show the revenues from iBooks, B&N, Kobo and Google Play but you can see how fortunes changed on Amazon over the course of the year.
So yeah. Maybe most underperforming series are best to stop and the author move on to something new. But my example shows it can be done and you can improve performance of a series with editing, new covers, blurbs, categories, keywords, ads, and promos plus finishing the series. You don't have to sink a lot of money into it and probably should be cautions, watching performance to see how it goes before doing a huge spend. Start minimally and increase as you see positive results.
