Sometimes when someone doesn't like a book, they can explain exactly why. They might have a problem with the content or the message. They may have thought the character motivations were unclear or preposterous. They could have found the writing style awkward or dense or simplistic. And sometimes, when this someone is a person evaluating your work, it can be helpful to you if they explain their why.
But the most difficult thing to hear might just be "I just didn't connect."
Sometimes this a euphemism for "I thought your book was terrible for X reason or Y reason, but I either can't or am not willing to relay those reasons to you, either for fear of hurting you or for fear of looking like a jerk (or both)."
But more often, it really is what it sounds like. The person just doesn't like books like yours. Maybe they prefer books they can personally relate to, and yours isn't relatable for them. Maybe they have trouble suspending disbelief for your science fiction concept, or don't find romances compelling, or can't drum up any enthusiasm for whether your fantasy novel's questing party can recover the world-saving artifact. Maybe they have a particular difficulty connecting to books that are written in third person, or in present tense.
And this "I just didn't connect to it" response goes all the way up to editors at big publishing houses and all the way down to readers deciding what to buy.
We all have preferences. It's doubtful that anyone who reads is going to say they are equally interested in all well-written books. And unless you personally continue to "give a fair chance" to books that open with action that doesn't grab you and books whose descriptions sound pretty boring to you, you should understand why this happens. Agents and editors frequently have to re-read books they sign multiple times, and both types of publishing professionals are gambling on whether readers will like what they like, because agents don't get paid at all unless they sell the book to a publisher and publishers won't make back the money they spent on producing the book if readers don't like it enough to pay money for it.
Publishing works the way it does because readers work the way they do. Except for situations like school reading or other assignments, we generally don't have any obligation to read things we don't want to read, and publishing industry professionals are in the same boat. If they don't want to read it, they probably won't be able to convince others that they want to read it.
So don't go into publishing, into story-writing, into creative careers in general if you think subjectivity doesn't or shouldn't exist. Don't expect "equal" or "fair" treatment, or for anyone to humor you. No one is obligated to read a predetermined amount of your book before deciding they don't want to read more, and no one is obligated to defend liking a book that you think isn't as good as yours. People will like things for reasons you don't agree with or don't understand or don't have in common with them. It could be you're writing stuff that not many people are interested in, or it could be that you're writing it in a way that doesn't make it easy to fall in love with, but either way if you're encountering lukewarm interest and "didn't connect" responses every time you attempt to engage readers, you might try asking yourself what makes YOU connect.
What is it about the books you love that made you keep reading them after page 1? What is it about the books you love that made them work so well for you? What was it about the books you love that makes you call them the books you love? How did those authors draw you in? You might think you're doing the same thing they are, or doing it as well as they are, but actually look at what they're doing. What do they do on page one? What do they do over the course of their story? What do your favorite authors have in common? You may be skipping some steps. You may be including or not including elements that turn people on or off.
Or you may be showing it to people who just didn't connect and there's nothing you can do about it.
You should always look at your material first, think about what you might be able to improve, but it isn't always your fault. It isn't always an actual flaw or problem with your work. You should be open to the idea that it might be, but not eternally convinced that it must be if some readers tell you they don't connect.
There isn't one right way to write a book, but if your first tries aren't getting you the results you want and you're determined to reach those results, try doing what worked for the authors you like. It works a lot better than blaming your audience or throwing up your hands and quitting. And it feels a lot better too. I promise.