If you have an indexer replication cluster, but see the search running only on the same indexers, then it can be that :
the ingestion of your data is not loadbalanced accross all indexers, so the same indexer always get the primary searchable buckets.
check on the cluster-master with the SOS app on the replication dashboard, where your searchable buckets are.
Or maybe did you setup site replication, and because of the search affinity the search-head always search on the searchable buckets that are on the indexers on the same site. So maybe always the same.