There should be two 'nuts' one is a locator and the other is the locker. If it is a 100mm grinder you can only use 100mm disks and nothing bigger. Make sure that the locking nut is placed on correctly and when you tighten it down, try to spin the wheel. If you can hand spin it, then you have either the locator or the locker on the wrong way.
[https://www.youtube.com/watch?v=KGkDcntMWMc|This youtube video has very good information for you.]