Switch between chat and presentation

While in a live class/meeting I’m sharing a ppt.
student sees the ppt fine. I also at this point add a poll question to check understanding.
Pupil clicks on the speech bubble so he can answer or  ask a question
then when he comes back from the chat he can no longer see the ppt.
This has happened numerous times. He then has to hang up and rejoin the meeting. How can I avoid it?

this student is on a mobile phone.