OPManager stops polling snmp for few devices - What do I have to check ?

OPManager stops polling snmp for few devices - What do I have to check ?

For few devices, (not linked) OPManager stops polling while we have no alert ! (we discover the fact by any chance)
For the majority of the other devices monitored, the snmp polling goes on.
For devices no more polled anymore, credentials are right and test passed.

OPManager Build Number 11600 / Windows 2008R2 / MSSQL 2008

How can I detect in the OPManager DataBase when it stops polling a device while we have no alert ?
(SNMP test passed // which table has to be verified ? what query may I do ? How can I repair ?)

What log file should I read to understand what happens ?

Regards,

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