Sometimes a story just doesn't click from the very first page, and that's fine. I don't think there's a single universal rule, but certain patterns make me pause. If I'm spending more time looking up world-specific jargon or complex magic system rules in the glossary than actually reading the narrative, that's a red flag. A dense lore dump with a dozen unfamiliar names, locations, and factions right away can feel more like homework. Another sign is when the action or emotional core of the scene gets completely lost because the prose is so convoluted trying to explain mechanics. You should be able to follow the basic 'who is doing what and why' even if the deeper intricacies aren't clear yet.
For me, it's also about balance. A great opening chapter can be challenging but rewarding because it makes you want to untangle its mysteries. A bad one just feels impenetrable, like the author forgot they need to build a bridge for the reader to cross. If I finish the chapter feeling more confused about the main character's immediate goal than intrigued by the world's potential, I might shelve it. That doesn't mean the book is bad, but it might require a level of focus I'm not prepared to give at that moment.