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.
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