You'll want to double check your VPN settings on both sides. There should be some clues in the event log that will point you in the right direction. https://documentation.meraki.com/MX-Z/Site-to-site_VPN/Troubleshooting_Non-Meraki_Site-to-site_VPN_P...
Here are some things you'll want to double check. If you are connecting to a non Meraki site then setup your MX as a Hub and configure the remote site down in the Non-Meraki VPN peers section.
Double check these things to make sure they match the far side
1. 'Private subnets' should be the same or in some case supernets of remote site
2. IPsec policies should match exactly
3. Preshared secret
4. All networks or apply to your internal networks
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO
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