All platform numbers at Euston are suppressed at the moment as the Darwin CIS web service client for Euston is disconnected and has been since 2nd September. This means the LICC at Euston is unable to tell Darwin to un-suppress platform numbers.
Other Network Rail managed stations (Birmingham New Street, Glasgow Central, Kings Cross, London Bridge, Leeds, Manchester Piccadilly and Paddington) appear to be disconnected too. This could be a software issue, or it could be a human issue. You might want to raise it with Network Rail.