Actors sometimes have trouble with their dialogue being understood. Maybe they’re running words together, or speaking too quickly, or not enunciating, or a hundred other things. There are two common solutions. One is internal, the other external.
Internal: Deepen your connection to the dialogue (and, of course, to the character and circumstances of the scene). In other words, internalize it more. Really absorb the dialogue’s meaning and emotion. What does it mean to you/your char? What’s the subtext? Why does your character say what s/he says? What does s/he want to emphasize — what words/phrases/meanings? Knowing the answers will automatically and unconsciously vary your pace, tone, and emphasis, all of which will make your delivery and enunciation more understandable.
External: Improve your enunciation. The best method: Use the classic text, Edith Skinner’s Speak with Distinction. It has LOTS of exercises for all aspects of enunciation. Find those that address your issue. Set your own goals, but thirty minutes day is a good start. They’re simple enough to do while driving, walking, cooking, etc. [Note: Practice NOT recommended while fooling around with your partner.]