Hi there! Is this in Factions or Survival? If it is in Factions make sure it is a /f baseclaim or it will naturally unclaim. If it is in Survival there shouldn't be any particular reason for your land unclaiming. If it continues please make a ticket in Discord or speak with a staff member in-game so we may better assist you.
Typically from the experience of myself and my fellow friends who play survival/factions; land unclaiming can glitch on occasion and happen if a player spends like several weeks offline or if there is another player with permissions tweaking with the claims and messing them up. If neither of those are the case, then I’d definitely recommend making a support ticket like Kaawaa said as they’ll be able to better investigate the issue and get it solved in a timely efficient manner. Hope this gets fixed for you soon!