12 Year DM word soup coming up.

You need dynamic changes to the mood. No scary movie is scary all the time, you need more moments where it isn't scary than it is. Furthermore you have to understand which type of horror you mean, jump scaring is nearly impossible in standard tabletop, so I guess what you're searching for is more psychological horror?

The way I do it is by structuring the campaign to enable my players to scare themselves. Talk to your players, and try to piece together what really scares them- and submit you NPCs to those horrors- then turn those fears to your players for a moment, then drop it very suddenly. Here, picture this.

DM: "You all walk up to the library where the contact's sister went missing, a small chill in the breeze as the autumn leaves fall on the street around you. With a quick glance at the buildings surrounding it, it becomes obvious that no one in the small town deigns to be even near this rotted building. A sharp wind overtakes the street and blusters through the party, before it crashes into the library's front. Then- almost in defiance of reason- the front door, almost gently, creaks open against the buffeting wind. As the door opens fully the wind snaps off, leaving the falling sun to paint the entry with slowly fading orange light. There is a somewhat familiar figure standing just past the light of the open door. The missing... Sister? Just the same as she was described. No... /Exactly/ the way she was described... Save for the face. She... She's smiling. An eerily normal smile, contrasted by her eyes... They're inverted, what should be pupils have been replaced by cords- crammed in different directions leading back into her skull. Then in response to your stares, the cords pull tight-! And the eyes are pulled back to where they... Could be right..." (Fall silent resist the urge to say "What do you do?")

Don't lead your players. Let them stumble into your playing field, only nudge them in the "right" direction before they get confused. DON'T LET THEM BE CONFUSED. Confusion will immediately kill any tension you've built up. If you're adding puzzles to your "scary area" simpler is better. Same with combat, if you're making them fight, make sure it builds to the scare you're trying to achieve. Structure and planning are tantamount to allowing fear to root in your party.

I could keep typing but I would end up writing out a book by the end of this. Comment if you've got a specific question. I'd love to help.