Hi 2700k. Have you signed in to the same Adobe ID on both the phone and the mac? Photos should automatically sync to/from the central repository in the cloud.
As Victoria stated: Both mobile device and Mac must be logged into the same Adobe account.
If you are using the Lr camera on the device, the sync process is automatic.
If you would like to use the device's native camera go into the Lr General settings on the device and enable Auto Add Photos.
Another possible cause: You mention 'on the go' implying you may be away from a Wi-Fi connection. If that is the case be sure
Use Cellular Data is enabled in Settings | Cloud Storage.
When you are sure photos should be in the process of syncing; on the mac desktop Lr ALT+CLICK on the cloud icon (upper right) and the screen will change to show photos being synced.
Another possibility is that in the LrCC IPhone App synced could be paused (cloud icon, top right). Although that would be quite clear.
Yes! And another little tip is to intentionally Pause Sync then Resume Sync. At times that clears a blockage that may be causing trouble.