following Setup

Logitech Media Server Version: 7.9.2 - 0023.1559540205 @ Wed Jun 5 14:51:56 CEST 2019
OS: Synology DiskStation - DE - utf8
Platform: x86_64-linux
Perl-Version: 5.24.0 - x86_64-linux
Audio::Scan: 0.95
IO::Socket::SSL: 2.037
Datenbankversion: DBD::SQLite 1.34_01 (sqlite
Number of Player: 3

2 PI 3B+ with pCP 6.0.0-b7

One of them (called Blau67) connected per ethernet with output on a BT Speaker. The BT Speaker is Player 3(called Boseplayer)
The other one (called WoZi67) connected per WLAN with output on a hifiberry DAC

I have configured a synchronisation between "WoZi67" and "Boseplayer", but this only works for an hour or so.
I start to play music and th sync is perfekt. After a while(sometimes 30 minutes, sometimes an hour or two hours the sync is lost and the only chance to sync it again is stop playing and start ist again.
Alternative i can power off/on the BT Speaker.

What can i do?