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.
NOTE2: You can add and port tftp at the end of your command to only limit network traffic to TFTP packages.