the missing data does not appear in the previous data received by SDLnet_TCP_Recv

The concept of a stream means that even though you may send your data with 1 call to 'send' you may require multiple 'recv' calls to get that data. Your issue sounds eerily familiar to some of my younger coworkers issues when moving an application from udp to tcp. They would make 1 call to 'recv' per iteration of the application loop assuming (incorrectly) that send/recv was a 1:1 operation. Locally on their work stations this worked fine until they started sending larger amounts of data per send call. Over a WAN connection the issue was even more apparent.

As Ohforf sake mentioned you will need to figure out some sort of protocol to let you know when you have received an entire "packet" or "message". Many older text based MUDs just used the newline as a delimiter for sending information to the server. More complex protocols will use a fixed sized header to define the size and type of payload to expect. Others (like HTML) will use a combination of delimiter patterns and headers to send/receive different payloads. I personally like to point beginners to the SFML sf::TcpSocket and sf::Packet implementations that handle this pretty simply and straight forward. I am not saying this is a great implementation (though many do much worse) but it is simple enough to get the point across.