I've often thought that the tensioners should not be a spring style tensioning arm. Instead they should be a clamping fixed arm that is adjustable. Also the arm should not hold the chain down at an angle but rather up toward the chain stay so that the wrap around the cog has more teeth in engagement.
In fact come to think of it perhaps you have too much chain so that the wrap isn't as long as it could be? Can you take out one more link pair and still fit the chain on? If it is that loose then you'll have a lot more wrap working for you.