#1 cause is user error on installation. Unzipping to the wrong spot.
#2 cause is user error, people using that hacked-up, cut-down derivative, SLOBS, not realizing that it's a crappier, less-capable fork of OBS Studio that should be avoided.
...that's really about it, except in cases like obs-ndi which require the right version of the NewTek redist to be installed as well. Or obs-virtualcam and not allowing it to install the virtual camera driver(s).
Beyond that we'd need specifics about which plugin, and preferably a logfile. Or just point the users here, and cut out the game of telephone.