I have not. I am new to using an EarthBox. I have grown vegys in other containers though.
I think the cover crop might be adding competition for the limited resources within the container.
It is my understanding that clover and such planted in garden or field soil help to trap nitrogen that would not be available to other plants, and then the cover crop is tilled into the soil to then get the captured nitrogen released so that the next crop can hopefully benefit from it. Potting mixes and container growing is very different than growing directly in the earth. I will keep an eye on this tread to see what others have to say....