I have this sometimes (today) and never really know 'how' I get around it (open a new instance on a slave - try to connect to it from Master and only see localhost - MASTER - listed as possible options to connect to). Save the project sometimes. Close and reopen project - and/or slave(s). Seems like one time one thing works - one time another. Today - so - nothing is working on 'seeing' this new instance.
Of course I have all slaves set up to 'advertize on network'. Am I missing anything here (whilst on am on the latest build - I have had the same issue from time to time on previous - otherwise working builds.)
Thanks in advance for any assist.