Panelist unable to join webinar using link in invitation email

I added 4 panelists to a webinar and sent them the panelist invitation email that included a link to the webinar. When trying to do a practice session, three of them were able to access rather straightforward, but one had trouble getting into the session. When I tested it with two personal emails, one opened while another never moved past "Please wait. The webinar will begin soon." Both of my personal tests were through gmail. Does this have something to do with Zoom on the panelist's computer? I don't see anything different on the backend of my Zoom webinar set up for this panelist vs. the others, nor between my two personal emails. How can I make it easy for this panelist to join? I do not have authentication required for panelists. He was eventually able to join but I'm not sure how he did it and don't want him to be unable to access on the actual webinar date.