Synch problems

I have a number of SB players & they all synch well together, via wireless.
However, I also have a PiCorePlayer streaming wirelessly to LMS 7.9.0 & I can never get it to synch to the others. It's always a fraction out.
I've read I can make adjustments to it by altering settings:
Player Start Delay (ms)

Player Audio Delay (ms)

Minimum Synchronization Adjustment (ms)

Network Packet Latency (ms)

but it doesn't make a difference. I've also tried adjusting these settings on all other players & leaving the Pi at default settings, but no joy.
It may be relevant that on the Information tab in Server Settings, I can see the wifi signal strength for all the players apart from the Pi, which is blank.

Please can anyone advise how to get it all in synch?

Many thanks
Paul