Ensure no traffic is being blocked towards the Cisco Meraki VPN Registries, you can find a complete list of IP addresses that must be reachable at all times for your dashboard to operate at its best, this list is on the top right of your dashboard under the menu Help > FIREWALL Info.
You can also go to your Security & SD-WAN > VPN status and check for any error messages, NAT unfriendly, or Connectivity issues to the VPN Registries.
If everything above looks good, and all checks have been made, you should then contact Cisco Meraki support with your findings and you will get all the assistance you need.
@SopheakMang, adding to what @Bettencourt stated, when your traffic is using ISP1 the MX450 is registering to the Meraki VPN Registry with the public IP address on ISP1, and using this IP address to build all the VPN tunnels. When you failover to ISP2 the MX450 first has to register its new IP address with the VPN Registry, then this gets sent to all the branch sites so the VPN tunnels can all get rebuilt on the new IP address. This really shouldn’t take long, but it’s not instant.
As @Bettencourt said, make sure all the firewall ports are open as required and see if there are any VPN Registry errors on the AutoVPN page - or and VPN failure events in the Event Log. If all is looking good you’ll need to open a support case as they’ll need to troubleshoot what’s going on with the VPN Registry and why the tunnels aren’t forming.