In Transnational Replication Subscriptions are not initializing

I have configured transaction replication in which there are 2 subscribers (S1 and S2) for a publisher (P1).

Subscriber S1 is working fine (all subscription syncing), it initialized all the snapshots P1 generated. After 3 days I generated a snapshot for all articles in P1 and added subscriber S2 to synchronize.

After 15 hours I went to verify whether S2 synchronized or not and I found the status of all the subscriptions of subscriber S2 is "UnInitialized Subscription".

I "Reinitialized the subscription with new snapshot" but after the snapshot is generation subscription status is same.

Looking for right solution.

Thanks