How are you triggering and what is the 70D set to? If it is set to E-TTL and you are using optical triggers, the exposure-setting pre-flash can cause naive third party flashes to trigger early.
While cheaper, third party flashes like this take a lot more time to set up properly. I suggest Syl Arena's Speedliter's handbook.
Hi I am triggering from the optical output from the built in flash. I have tried on manual as well as various other shooting modes.
I have tried it with E-TTL and without. I'm using back button focus so when using E-TTL and I focus a pre flash is seen. Then taking the shot the flash does trigger but cannot be seen in the image.
It's just so annoying, it's obviously communicating OK with the flash unit, but just not in sync.
Very confused and frustrated.
I'm probably grasping at straws, but is it possible that you're using 2nd-curtain sync? If you were, maybe it's conceivable that the shutter closes before the remote flash gets triggered. Far-fetched, I know, but when all else fails, ...
What are you using as your "budget set of portable flash"? Is it compatible as an optical slave? It appears that either the settings or the flash itself is not compatible with the camera.
Since you stated that the remote flash fired but its output is not registered. That tells me that it went off after the camera has already taken the shot. If the portable flash is based on light alone, this might happen. You might want to set the shutter speed to something long, like half a second to see if the flash would be registered.
For optical slave (which is suitable for using the 70D built-in flash as optical master), I find the most reliable is of course a Canon flash. Short of that, try a Yongnuo flash...it's highly compatible.
Do you or someone else have another DSLR that you can try the slave flashes with? For that matter, even a point and shoot camera with a flash should fire the slaves if they're optically triggered. It sounds like you have two or more of the slave flashes. Have you tried triggering them individually? If you're setting the camera manually, what shutter speed are you using?
What brand and model are the slave flash units you're using? Maybe someone else here has some previuos experience with them that they can share.