Without conflict there is no story. No matter what the story is, you can boil it down to “The protagonist wants something and someone or something is standing in the way.” The novel is about getting past that person or thing to resolve the conflict and earn the protagonist their goal.
As simple as that sounds, it’s not always so clear when you’re looking at your idea. Ideas are often more concept than plot, a great premise without a solid story yet, and the conflict at the core of that idea is fuzzy. Your instinct tells you it’s there, but critique partners or even agents just aren’t seeing it.
In most cases, the conflict isn’t strong enough yet, or it’s not clear what the actual conflict (and problem) is.