UDP is a datagram-based transport. There is a 1:1 relationship between the size of the data being sent and the size of the data being received. The default value of the TIdUDPServer.BufferSize property is 8192 bytes, so right there, that is not enough buffer space to receive 9000 characters. You need to increase the BufferSize.
The TIdUDPServer.OnUDPRead event is only fired for datagrams that are received in full, partial datagrams that get truncated due to lack of buffer space are ignored and discarded.
What is the actual encoding of the strings you are sending? Using a packet sniffer, like Wireshark, what is the actual byte size of the datagrams that are not being received correctly? UDP has a maximum payload size of 65507 bytes.
The TIdUDPServer.OnUDPRead event is only fired for datagrams that are received in full, partial datagrams that get truncated due to lack of buffer space are ignored and discarded.
What is the actual encoding of the strings you are sending? Using a packet sniffer, like Wireshark, what is the actual byte size of the datagrams that are not being received correctly? UDP has a maximum payload size of 65507 bytes.