NTP mystery

'ere’s a complete puzzle.

Three RB112’s making up a sub-net all with RouterOS 2.9.34 installed. All as far as I can tell identically configured. All three are set with their NTP client pointed at a NTP server for the network but only one will synchronise. The other two will start but not even reach the server. However if I point them at a NTP server off the network they will synchronise.

The only difference between the three I can think of is that with the two that can’t connect to the local (MikrotikOS) NTP server the NTP package was loaded with the rest of the 2.9.34 update but not enabled until later. The one that can connect locally was installed and enabled simultaneously.

Any ideas?