u/Jc_Not_Caster

So me and my friends have a server, its only me and them. (so 4 people) Recently i went into a cave to mine some stuff but sometimes we find areas where we cant break the blocks (we mine them and then they reappear like with server desync) none of us use the claim option and i dont think its a issue with the server because we dont have trouble with connecting, we play on deceasedcraft, anyone know if it could be a problem with the modpack?