Unable to admit meeting participant from host window

[SaraMac]

I am using Zoom One Pro.  I use it for one to one meetings.  I have found quite often I start the meeting as host and am waiting on zoom and am unable to admit the participant from that window.  I receive a message in my email informing me the person is in the waiting room and then have to close down the meeting I was waiting on and re-open it from the link in the email.  This means that the meeting starts late.  The confusing thing is that sometimes it does allow me to admit the other from the host window I've already opened.  Can anyone advise me what is going wrong when this is not enabled?