Have you configured the root priority for the switch that should be the core of your network?
https://documentation.meraki.com/MS/Other_Topics/Switch_Settings#Setting_the_STP_Root_Bridge
Next the other switches, what are they? If they are Cisco Enterprise switches I would configure them to use mst "spanning-tree mode mst".
If they are dumb layer 2 switches it is possible they don't even run spanning tree.
Look for links between edge/leaf switches that should be there. You ideally want a loop free design where the downstream switches only connect to the core switch and no other switches.