Prometheus cluster metrics appear in Explore tab but not in dashboard variable


A number of my Prometheus clusters don’t appear in my dashboard variable (there should be 13 in total)


but metrics are available in the explore tab image

if i run sum(namespace_pod_name_container_name:container_cpu_usage_seconds_total:sum_rate) by (cluster, namespace) in Explore I can see all clusters.

Any ideas whats causing this?