It sounds like the opposite may be happening, the gas is too strong pressure for the oxy. A flash back arrestor isn't needed with oxycons as the airflow isn't stong enough to open the valve (keep one on the propane line for sure!). There is no oxy under pressure (tanked) so one isn't needed for safety reasons, and another plus, it's a surface mix torch.
Having several check valves in the line probably is restricting the air flow as well. I'm not going to tell you to remove all the check valves, but I will tell you I have none on mine. I'm not expert in the matter, it's just worked for me for years.
This video may give you a bit of an idea. I use the plastic tees instead of a manifold, but you can see he has no check valves or arrestors.
https://www.youtube.com/watch?v=WDkhQXKY0K8