This problem is usualy very network specific. All we can say for sure is, that the snmp request is not answered within the snmp timeout period.
You can try to increase the timeout. The setting can be found in the device or group settings in the categorie "Credetials for SNMP". The setting is called *SNMP Timeout (sec.)*. This will only help you, if the problem is a too slow response during "heavy load". If your connections fails because of any other networking problem, like for example a switch reboot, maintenance or something like this, increasing your timeout will not help. You could try to add a ping sensor and check, if the ping sensor shows higher pings or failes, at the same time as the snmp sensors.