KVM (Pro) - blank Secondary on Primary computer

Hi folks

I'm looking for a steer on why I'm no longer able to view the Secondary computer on my main Primary PC.  The screen is displaying just a big square of grey!

A quick summary:

• I can successfully ping the Secondary from the Primary and vice versa
• When on the Primary, if I click on Configure / KVM and then Edit (below the list of 'other' computers) and then click on the cog icon button with the green tick it states that it was able to connect successfully
• The multiplicity icon is displayed in blue on the secondary but is grey on the primary
• The various passcodes and encryption keys match
• The configuration screen on the Secondary flickers every 4 or 5 seconds which indicates to me that something is trying to connect but nothing is happening
• I have a new ASUS RT-AC87U wireless router and I'd set up Wireless Mac Address filtering to only accept the devices on my local subnet but I've since dumped all this in case it wasn't happy for some reason
• I'd also set up DHCP to assign a particular IP address to the Primary and Secondary PCs (40 and 30 respectively) but again I've since removed this setting and let the router allocate them automatically

Any idea what might be the problem here?  Any suggestions would be very welcome!

Thanks.