I think you need to give them time to prepare what they say. A well-prepared discussion is usually more successful with low level students. You say yourself “they do not speak English well” and you need to bear this in mind. Students with low level speaking ability find discussions very difficult.
Preparation can be done in groups, pairs or alone. It can be done in writing or orally. One idea is to give your students phrases which express agreement and disagreement with the topic; e.g. crime on TV generates violence; TV has little effect on violence, etc. Students organise the phrases into agreement or disagreement and strength of opinion (mild agreement, strong disagreement, etc). You can then practise the use of phrases (e.g. which form of the verb or preposition follows), and their pronunciation. Students can then prepare what they will say using the phrases you have just worked on.
Think carefully about the topics you use. Try to choose things your students know about and are interested in. Work first of all with what they enjoy. Think about what they would discuss naturally with their peers.
Remember to encourage and praise all efforts, even short sentences. If they say something like “yes, it is bad” at least they are showing they have understood. With praise and encouragement, they are more likely to try again. The more practise they get, the more they will reinforce what they know.
I hope this will help.