Hi Gerald,
SNMP contact lost might happen for below issues.
1) Communication error because of routing or blocking by firewall , you may be able to ping so please check the firewall rules.
2) SNMP profile configuration- right click on the device go to MIB tools and check the new window status , there we can see the device down reason with error message.
3) Java console screen has console tab where you can below error message for the device which has issue.
SNMP Contact Lost: No SNMP reply from device x.x.x.x caused by SNMP Error: Unknown User Name[-33], last uptime was 10 Days 17:54:18
Even after checking the communication and configuration if you still have any issues please let us know. we can open a GTAC case and check the packets between both switch and Netsight interface.