It sounds like we should get a ticket registered and collect logs to see what is going on. Are direct help desk number is 1-877-459-7304 ext 4310039. They will be able to help collect logs from your server and an effected endpoint to see why they are falling out of view even though they've checked in within 180 days.
PG Software & Solutions IPS
Dell Data Security
Need Immediate help? Please call DDS Support @ +1.877.459.7304 Ext. 4310039