Daily user of other video conferencing with high speed internet cannot get into Meet Jitsi

I’m running a class for 6 people for 16 meetings, with 12 remaining. One is an engineer who meets daily in other video conferencing services for his work. But he cannot get a good connection to Meet Jitsi. Tonight he said he had a message that Jitsi couldn’t pick up his audio. He has been getting in via telephone. He says Jitsi is the only video conferencing he has trouble with. Anything I can suggest to him to do/try?