Throw obstacles in your character’s path.
Ask yourself: What is the absolute worst thing that could happen to my character right now? And then have it happen. This is something Stephen King does so well (though, quite honestly, I don’t think he ever gets stuck). In King’s novel, Mr. Mercedes, the protagonist strikes up a friendship with a young neighbor. The neighbor has a cute sister who wants to go to a concert. Suffice it to say, that girl gets into terrible danger, and King draws it out so that by the time the whole thing is over, you’ve just about chewed off your fingernails. But you don’t need to be writing horror to do this. Let’s go back to our romantic Molly. What’s the worst thing that could happen to her? Maybe she meets a man right before her 30th birthday, but it winds up being her best friend’s new boyfriend. Or he’s about to be shipped overseas. Or (because I’m a mystery writer) he’s a killer. Or he has red hair and she’s always sworn not to marry a man with red hair. Or his mother’s in jail. I could go on forever. Just keep in mind that the more you raise the stakes, the more things you throw at your protagonist, the more quickly the story will move forward. Not only are obstacles interesting, but they also make characters change and grow. They inspire us. They inspire the writer. You’ll be amazed at the things you can come up with when you put your character in a corner.