Have you had the opportunity to speak to both those students when they are alone, rather than in a group situation? Sometimes it is the presence of other learners which causes people to freeze. They assume that all the other students are understanding more and that their own English is the lowest standard in the group. Sometimes, in a one-to-one situation, they relax and open up.
When I taught in Madrid, I gave private classes to a couple (a man and woman who lived together). Our classes were mostly conversation-based and, despite my best attempts, the lady did almost all the talking with her boyfriend only speaking when I pointedly (and politely) asked the lady to stop talking! One week, the lady was ill and rang to cancel. She told me that her boyfriend would still attend. I was quite surprised and very interested to see what would happen. It was a revelation - he was talkative, interesting, funny and completely relaxed. It was as if a totally different man had come to class. At the end of the class, I asked him how he felt about the class. He immediately said that he had really enjoyed it but also admitted that he wasn't really enjoying the joint classes and had been thinking of dropping out. When i asked him why, he said that he knew his girlfriend's English was better than his (true) and that made him a little embarrassed about speaking in front of her. He also said that he thought I would find it more interesting and fun to talk to his girlfriend so he kept quiet. I told him that I sympathised with how he felt but reminded him that the only way his English would improve was if he practiced and joined in with the classes. At the next class with both of them, something interesting happened. I asked a question at the beginning of the class and the lady began to speak. She had only said a few words before the man said to her "I'd really like to answer this first" and he then talked for about two minutes without stopping. His girlfriend sat back, listened and looked rather happy. At the end of the class, while he went to the toilet, she told me that she had been really pleased when he started talking - she said that she had been desperately trying to talk as much as possible in previous classes specifically because she knew her English was better and she didn't want him to be embarrassed! It was a great example of lack of communication - between all three of us, admittedly.
I didn't mean for that to turn into such a long story but you never know, you might find something useful in it.
Edit: Extra question. You said that they find it impossible to focus on the spoken word. How is their reading comprehension?
Retired English Teacher