@Wayne-TP Thanks for the response. My firmware is always kept completely up to date. Predominately it seems to be the SubG devices but I think I had an issue on one occasion where the C420S2 cameras weren't triggering a hub chime when the other devices weren't working either. I've even tried moving my Hubs to the top 2 priority devices for my modem just in case that could have been an issue but it's still occurring. 
 

What could cause the issues whereby the app activates plugs, bulbs, etc instantly while at the same time, the motion sensors and smart buttons are greatly delayed or just won't trigger the devices at all?

  @Wayne-TP Thanks I think I understand what you mean now. I do have P100/P110 automations setup already (i.e. schedules). Given that this seems to be a sporadic intermittent issue, it means it would need to occur during one of the occasions where the plugs are being activated. I think from memory that happened a few days ago where one of the plugs didn't activate at the specified time. I also just got the support email so will follow-up with them. Thanks again.