Lightroom and Lightroom Classic don't sync

Hello everybody. for a few weeks now the photos in my lightroom no longer synchronize with the Lightroom Classic catalog. in Lightroom I have several albums of more than 2000 photos all edited and flagged, these changes are no longer reflected even on the respective photos in Lightroom Classic. The special thing is that on Lightroom it tells me that the sync is complete.
how can everything be re-synchronized?