Q: Why does talk text only work sometimes?????  At times it works fine and then it will either show me the blue circles forever or just stop without converting anything I said.

Probably because Siri is still in BETA form.

beta is testing.  a prerelease.  siri has a learning ability.  the more you use it the better it becomes.  i needs to learn your speach for you.  the more you use it the better it will be. 

The trascription doesn't take place on your phone. The audio file is processed a little on the phone and then sent to Apple's servers for the bulk of the work, and the resulting text is sent back to your phone.

Sometimes there might be problems connecting to the internet to send the file. Sometimes there might be problems with Apple's servers in a specific region. In my case if I go outside my house a bit my phone will still be trying to connect to the wifi (there is just a little bit of a signal). There isn't enough wifi to handle sending the audio files to Apple, but enough that the phone keeps trying to use it.

Things that sometimes help. Turn off wifi if it is weak. Put your phone into airplane mode for a few seconds and then turn the radio back on.

If the problem is at Apple's end. Just wait a while.