Sounds like it could be a torque problem, but based on the fact that the ratio of the previous drive had less torque and worked fine, it could be a chain issue.
So about the chain- contrary to what most people are saying, this does not look to be primarily an issue of having too much slack (although, that could very well also be the case), but rather that the chain does not have a high enough wrap angle with the sprocket. Allow me to explain.
This machine’s chain drive illustrates the proper use of a tensioner and sprockets with adequate wrap angle. With more wrap angle, you get more contact between the chain and the sprocket, thus mitigating the possibility of slippage and ensuring that all the torque is transferred properly.
I would recommend that you increase the distance between the center of the sprocket and other two until you have a wrap angle of at least 135° (looks like yours is around 110° or so at the moment) to allow for the driving sprocket to properly transfer power between the two driven sprockets.
Check out this ANSI chain selection guide for more information on chain drives (full disclaimer- vex doesn’t follow normal industrial chain and sprocket standards so you may want some extra wrap angle than what they recommend to be safe):