Connection problem

I have Multiplicity on two computers side by side.

Seamless on primary shows 'This computer' with black mouse.

Secondary shows computer name (correct) and IP address (correct)

and 'Enter this information on the primary'

Back to the primary, click 'Add computer' Enter the name here' 'or select from the following computers'

'Add computer box' gets stuck on --= Scanning =--  not finding the secondary.

manual entry of computer name and passcode saved.

settings test icon clicked and this pops up:

All the above checked. No obvious problems. Ping succeeds on both IP addresses. Computers share files and mapped drives OK.

I tried letting Multi through the firewall but it didn't help. Something else does not compute...