UDP does not guarantee delivery, it could be that the Java UDP buffer overflows because the application does not read the packets from the Socket fast enough. When a UDP buffer overflows, the Java Runtime (i.e. the operating system) drop any new packet.
SNMP4J 1.11.5 is quite old and you should anyway upgrade to a new version.
But in any case, you need to analyse if there is actually a buffer overflow happening (what is very likely but not 100% sure). You may, for example increase the buffer size in your operating system.