Hi Mohit,

Streaming API doesn’t guarantee durability and reliable delivery of notifications. Streaming servers don’t maintain any client state and don’t keep track of what’s delivered. The client may not receive messages for a variety of reasons, including: - When a client first subscribes or reconnects, it doesn’t receive messages that were processed while it wasn’t subscribed to the channel. - If a client disconnects and starts a new handshake, it may be working with a different application server, so it receives only new messages from that point on. Some events may be dropped if the system is being heavily used. - If an application server is stopped, all the messages being processed but not yet sent are lost. Any clients connected to that application server are disconnected. To receive notifications, the client must reconnect and subscribe to the topic channel.

Reference:
https://developer.salesforce.com/docs/atlas.en-us.api_streaming.meta/api_streaming/intro_stream.htm
https://salesforce.stackexchange.com/questions/53522/streaming-api-channel-getting-disconnected-after-certain-interval
https://salesforce.stackexchange.com/questions/213107/platform-events-length-of-time-an-event-is-published-for

Thanks,