SWGT has a built in security mechanism where if a player not in an enabled guild and does not participate in a guild war, siege, world guild battle or labyrinth in a set time frame (configurable in your site setup), then the player JSON data is purged from the system.
Monster subjugation is not currently considered as activity due to Com2us data restrictions.
For existing members, make sure you are following the SWGT guide to update your guild member list, as well as sync guild war, siege, world guild battle and/or labyrinth data regularly.
For new guild members, accept them through their recruitment application and create their SWGT user account through the accept feature. This will give the player 7 days of immunity from JSON data purging.