ST-E3-RT not firing groups simultaneously?

[MojoDK]

Hi

I have two 600EX-RT on my ST-E3-RT. 

When both flashes is set to group A, they fire simultaneously.

If I set one flash to group A and another to group B, then group B flashes a half second later.

Can't figure out if I touched a setting on the ST-E3-RT. I've tried clearing it, but it is the same.

Any idea?

Thanks

[diverhank]

Can you specify what modes and what the settings are?  That will help narrow it down.