recvfrom contiene exceso de bytes

This is a multi-client socket program.

When I am trying to run my program the few last recvfrom() do not return any data. But the recvfrom before that have the excess data. Can someone help me how to fix this.

Cliente:

while (1) {
memset(buff, 0, sizeof(buff));  
recvfrom(sockfd,buff,sizeof(buff),0,(struct sockaddr *)&servaddr,&len);
printf("%s\n", buff);
if (buff[0] == 'U') {
    while(1) {
        printf("Insert your username: ");
        fgets(username,sizeof(username),stdin);
        username[strlen(username) - 1] = '\0';
        printf("Username chosen is %s\n", username);
        // Username Check (Error Check)
        if (strlen(username) < 1 || strlen(username) > 16) {
            printf("Minimum of 1 and maximum of 16.\n");
            continue;
        }
        for (i = 0; i < strlen(username); i++) {
            if (isalnum(username[i]) == 0) {
                printf("Username must contain only alphanumeric characters.\n");
                j = 0;
                break;
            }
        }
        if (j = 0) {
            continue;   
        } else {
            break;
        }       
        }
        sendto(sockfd,username,strlen(username),0,(const struct sockaddr *)&servaddr,len);

        memset(username, 0, sizeof(username));  

    } else {
        break;
    }
}               

printf("Players:\n");

memset(buff, 0, sizeof(buff));  

for (i = 0; i < numplayer; i++) {
    printf("BUFF: %s\n", buff);
    recvfrom(sockfd,buff,sizeof(buff),0,(struct sockaddr *)&servaddr,&len);
    printf("%s\n", buff);

    memset(buff, 0, sizeof(buff));  


}       

Servidor:

      for (i = 0; i < numplayer; i++) {
    while (1) {
        memset(mesg,0,sizeof(mesg));
        if (recvfrom(connfd[i],mesg,sizeof(mesg),0,(struct sockaddr *)&cliaddr,&len) < 0) {
            perror("Recvfrom");
            exit(-1);
        }
        for (j = 0; j < numplayer; j++) {
            if (strcmp(username[j], mesg) == 0) {
                // Reinitialize buff
                memset(buff, 0, sizeof(buff));  
                check = 1;          
                break;
            }
        }

        if (check == 1) {
            check = 0;
            sprintf(buff, "Username already exist!");
            if ( sendto(connfd[i],buff,strlen(buff),0,(const struct sockaddr *)&cliaddr,len) < 0 ) {
                perror("Sendto");
                exit(-1);
            }
            memset(buff, 0, sizeof(buff));  

            continue;
        } else {
            sprintf(buff, "Valid Username!");
            if ( sendto(connfd[i],buff,strlen(buff),0,(const struct sockaddr *)&cliaddr,len) < 0 ) {
                perror("Sendto");
                exit(-1);
            }
            strcpy(username[i], mesg);
            printf("Username of Player %d is %s.\n" ,i + 1,username[i]);
            break;  
        }
    }
}
    printf("Players:");
for (i = 0; i < numplayer; i++) {
    memset(buff, 0, sizeof(buff));  

    sprintf(buff, "> %s", username[i]);
    printf("%s\n", buff);   

    for (j = 0; j < numplayer; j++) {
        if ( sendto(connfd[j],buff,strlen(buff),0,(const struct sockaddr *)&cliaddr,len) < 0 ) {
            perror("Sendto");
            exit(-1);
        }
    }
}