title: slack channel_not_found when sending message
body: <p>My workspace is naveedapex.slack.com and I have created two app "testslack" with bot user named 'ubot' and "user4k" with bot user named '4k'.</p>
  
  <p>I have three member in workspace: 1) naveedjb 2) naveeditv 3) naveedoff</p>
  
  <p>When users (naveeditv and naveedoff) send message to bot '4K' then Events API send event to given url where a message is being sent to channel DAD18LNBF (which is provided by EVENTS API) using <code>chat.postMessage</code> Method of Web API but ERROR arose </p>
  
  <pre><code>{ 
     ok: false,
     error: 'channel_not_found',
     acceptedScopes: [ 'chat:write:bot' ] } 
  }
  </code></pre>
  
  <p>But when naveedjb send message to bot '4k' then HEN EVENTS API send event to given url where a message is being sent to channel DAD18LNBF (which is provided by EVENTS API) using <code>chat.postMessage</code> Method of Web API but NO Error is coming and message delivered successfully</p>
  
  <p>Why message is not being sent by first two user?</p>