[emile]

The way you described it, you are asking them questions and expecting long answers, which may not seem appropriate to them in that situation. I would suggest giving more peer-to-peer work, such as conversation worksheets, and leaving them to it. With a small group of four, you could even leave the room altogether if they promise to complete the task in English.

The other suggestion is just to sit down with them and explain in detail why it's important for them to speak in complete sentences. Maybe you could talk about turn-taking in speech and communication skills, show some video clips.

some conversation worksheets