The problems with bad summaries generally fall into three categories: (1) confusing, (2) rambling, and (3) below minimal.
(1) Most indie authors use rambling summaries for their books. These go on for 6+ paragraphs. Instead of hitting the main points, they add in a bunch of unnecessary details. Summaries should answer the basic: Who? What? When? Where? questions. Who is your book about? When and where does it take place? What is the major conflict (not all the subplots)? It shouldn't take more than a couple of sentences to answer each of these questions.
The conflict is important for a summary because that is the main plot you are selling to your audience. All fiction books need to have a central conflict if the author wants them to be well-received by the audience--even if that conflict is minor in terms of world events (i.e., how does the main character bridge the gap between his daydreams and reality). Non-fiction books often serve a purpose that is useful, which takes the place of conflict--but this purpose needs to be in the summary, too. For example, a book about raised-bed gardening would explain to its readers how to do this. Other non-fiction books, such as biographies or memoirs are interesting because they detail someone's life--and hopefully included the conflicts that person had.
(2) Authors who haven't really thought about what their books are about tend to write extremely confusing summaries that also tend to ramble. They pick out subplots instead of identifying the unifying plot. One summary by an indie author, for example, talked about the main female character going on the run with the main male character because she saved a dog, and then the male was framed for a crime committed by a prince. It then talks about miners--without explaining how they are significant to the main characters being chased--and a princess who has some undisclosed bad thing happen to her. It goes on to say astronomers and a named bad guy (who is not the guy chasing the couple) see something in the stars, and the bad guy starts murdering people so he can be emperor. Huh? Since none of the above is connected, you probably think I left something out, but that is all the information you get from the indie author's summary. This book has good reviews so far, but I struggle to believe the author can keep all the plots in the book together since the summary is so disjointed.
I think confusing summaries are also because authors want to make their descriptions "cryptic." They are not merely keeping the ending from readers. Instead, they are not giving us enough of the correct information. Since two bad guys are mentioned in the above summary, I have no idea who is the real bad guy or how either are connected. I also have no idea what the major conflict is? I suspect the real summary should have told readers who should have been the ruler, who was the ruler, who wants to be the ruler, and who is going to try to get the right ruler on the throne. You can summarize what characters are trying to do without telling the reader if they will succeed or not.
(3)Which brings me to the last point: the less than minimum summary. It's important to not give away the conflict resolution in your book when writing a summary. People often do this by being so ambiguous readers don't really know what the book is about. This usually confuses readers, too, but it can be clear without telling the reader important things that will help him/her select a book. A less than minimum summary would be:
This amazing historical romance has Jamie and Dalton struggling to fall in love after they meet. Read it to find out if they overcome the oppression!
A better summary would be:
Jamie and Dalton both went to their 1953 Senior Prom with the wrong date and then married them. Years later, they return to their hometowns with a divorce under their belts and excess baggage. When they are recruited to be chaperones for their Alma Mater's Senior Prom will they finally end up leaving with each other?
