Multiplayer Game

Please advise why there is still so much inconsistency with being able to play multiplayer games.    In my class of 30, there is typically anywhere from 2-10 students unable to join at any given time.  Some days it is just one group unable to join multiplayer games sitting right next to one another (not that it matters, but literally one the same wifi and router).  Today I had a student at home send the code and had his group in class join...no problems. We have done all the troubleshooting suggested via the help.  I have such a great unit I use with the students, but the real power comes from watching them work collaboratively to complete a huge project, and that is lost almost everyday (replaced with frustration).