Teams meetings

hi,

I had a problem in few months: I can't attend the meetings if they start with the option "immediately" and I can't attend without being invited by someone already connected to the meeting. This problem doesn't arise when the meeting is scheduled. I need to solve this problem as soon as possible because I cannot continue this way, everytime I need to ask to my teacher and my classmates to call me and this situation is starting to annoy everyone, me in primis. 

thanks