STudents cannot join my regular meeting sessions via Zoom

If they are seeing that you're already in the meeting, there likely is a waiting room on the meeting. While in the meeting, go to Host Tools > Enable/Disable waiting room to turn it off. Conversely, you can let your students enter the meeting from the waiting room under the Participants area. 

Hope this helps!

We are also having issues with students who are unable to join calls using the regular passcode. Many have had to try the passcode multiple times and finally got "lucky" while others can't get in at all. It is the same passcode we've been using - it's just suddenly troublesome. This began with the update that was pushed recently. Any ideas? 

We typically embed the passcode to avoid any confusion. Additional we send reminders for all our Zoom meetings using Salepager (a Zoom app) that includes all the relevant information including passcodes, phone numbers, links to make it easy to join the meeting.