Weak dialogue usually isn't a writing-skill problem. It's a subtext and voice problem. Here's how to diagnose and fix it.
Real speech and good dialogue aren't the same thing
A common misconception is that natural dialogue means transcribing how people actually talk, filled with filler words, tangents, and pleasantries. On the page, that reads as flat and slow. Good screenplay dialogue is a compressed, purposeful version of speech: it sounds natural while doing far more work per line than real conversation does.
Write for subtext, not statement
Dialogue that states exactly what a character thinks or feels tends to read as flat, because it removes the audience's job of interpreting the scene. Strong dialogue is usually about something other than what it's literally about: characters talking around a conflict, avoiding a subject, or saying one thing while clearly meaning another.
Give each character a distinct voice
If you can't tell who's speaking with the character names covered, that's usually a sign every character is speaking in the writer's voice rather than their own. Voice comes from specifics: vocabulary, rhythm, what a character notices, what they avoid saying, and how directly or indirectly they get to a point.
Cut ruthlessly in revision
First-draft dialogue is often longer than it needs to be, with characters explaining things the audience already understands, or restating what action just showed. In revision, look for lines that can be cut entirely, replaced with a look or an action, or trimmed to their sharpest version without losing the point of the scene.