When a failed real estate agent is threatened to be fired, he must sell a house no one else has succeeded in selling.

Never start a logline with “When.” It’s like burying the lead. No one cares about things that happen; we care about the people to whom those things happen.

Past that, the premise isn’t clear. If he’s already a failed agent at the beginning, why is he still working at the agency and attempting to sell a house? Maybe a different adjective or tense would help, like failing, or down on his luck…or if he’s young and new to the industry, trying to succeed and/or thrive, he could be a junior agent, or maybe a senior agent trying to sell a house he never could, do so before forced retirement…I’m just not seeing the stakes here. “Threatened to be fired” isn’t that much of a threat. People get fired all the time. In fact a lot of characters get fired in the first act so they can pursue what the movie’s really about in the second act.

Basically, you haven’t made your story clear, so there’s no way any of us can help improve your logline because we don’t know enough of the details behind it. Why has no one sold the house? Is it haunted? Is it a shithole? Too gaudy, too expensive, too rural, what? It doesn’t sound like there’s enough conflict to sustain a feature, unless there’s more detail you haven’t hinted at. A logline has to give an impression of the scope of a story without specifying or giving away all the details.