First thing that you should do is to ensure your TFTP server is actually working and ensure it is configured for logging. This way you can check if RPi `TFTP` requests where received by `TFTP` deamon.
`TFTP` uses `UDP` and so is not really reliable. You can debug it by capturing the network traffic on your TFTP host using `tcpdump` or `wireshark`:

For `tcpdump`, you can use something like (you can termianate it by CTRL+C)

    sudo -i tcpdump host 192.168.2.2

Where `192.168.2.2` is your RPi IP address. You should see all the network packages sent to/from RPi. This way you can see if `TFTP` packages are sent at all. 

NOTE: You may need to use `-i interface` (before word `host`) option to specify interface other than default if you have more than one on your server.