What de-brick guides are you looking at? There is (sometimes) a very small window to TFTP with the device starts up, may be why you got the response just that one time. When I bricked my WRT54G, I had to have 4 ping windows open to be able to catch a response, and initiate the TFTP transfer to the device. If it doesn't hear anything in a second or two, it continues booting.
You'll obviously need to statically assign your computer to an address. Get a few ping windows going, and boot the device. Plug into the LAN port, I always do the first port, because I thought I read somewhere once that it mattered, though I'm sure it doesn't, but can't hurt. If it responds even once, you may have a chance.
Also, some devices have a recovery mode that they can boot into by holding the reset button while powering on the device. Some even have just a basic page with a firmware upload box. So, if the fist option doesn't work, try holding the reset switch and turning the device on, and see if you get any ping love there. if you do, try and get to the page. I think this page only stays for a short while too, so if you don't get it on the first try, try again. Or try TFTP this way.
These are just the basic steps I try when I think something went sideways somewhere. A guide for your particular device would be much better in the long run, because people have tried all these and more and found what works and what doesn't. Good luck dawg.