Readers get disgruntled when they feel like they’ve put more effort into reading your book than they got pleasure out of it.
(It’s always important to bear in mind, when you’re thinking about this, that there are all sorts of readers deriving all sorts of pleasure from what they read, and you have to be able to distinguish between genuinely disgruntled members of your own audience and readers who are disgruntled because your book wasn’t written for them. The latter aren’t your problem, no matter how much they may sound like it sometimes; the former are your problem, because you’ve failed them somehow — and while you probably can’t fix it in the book they’re unhappy about, you can try to do better in the next one.)
Anyway. A common source of the more-effort-than-pleasure problem is unsatisfying characters. The need for satisfying characters sometimes gets mistranslated as a demand for likeable characters, or for admirable ones (the phrase “positive role model” comes into play a lot here), or for ones with which the reader can identify. In fact, the reader will happily follow along after a character who is none of these things — an unlikeable scoundrel who has little or nothing in common with the reader — so long as that character is interesting. An interesting villain will hold the reader’s attention better than a boring hero, any day of the week.
How do you make a character interesting? That’s a bigger problem than a single post can handle, but here’s one idea for a start: give your character important things to do, and have him or her actually do them. A proactive character is an object in motion, and objects in motion draw interest.