First, Drag and Drop is not available in Multiplicity KM - only available in KVM and greater. Also, while Drag and Drop is available on on the KVM version, it does not work for KVM sessions, only Seamless. You would use Copy / Paste for KVM sessions.
Please make sure you have rebooted each MP PC to see if the problem lasts past it. You also want to ensure that each MP pc is running the exact same version ('About' tab)
If that fails, this is almost always a firewall issue. As a test, on ALL MP PCs, turn off any and all AV \ Firewalls including Windows native:
After, make the connections again to see if mouse and keyboard actions are observed on any Secondary
If it works, turn the Windows firewall on, download the following file (on all MP PCs), and run the batch file:
https://cdn.stardock.us/support/uploads/MPPortFix.zip
That file will only work for Windows firewall. For any third party ones, here are manual instructions:
Make sure these firewall rules are enabled:
Rule Name Port/.exe
Multiplicity Service 30564
Multiplicity Clipboard 30565
Multiplicity Application (clipboard) Multipl2.EXE
Multiplicity Audio Service 30567
Multiplicity Remote Screen Service 30569 (For Multiplicity 3 only)
Multiplicity Application (Remote screen control
MPRDP64.EXE (For Multiplicity 3 only)
All ports are TCP. The executable names above are for whitelisting the applications within a software based firewall (Windows Firewall for example).