Set the responce channel of the actor you want to find to OVERLAP, not to BLOCK.
Explanation: If you cast a multy trace (sphere or box) to a specific channel, given a START and an END position (those two position cannot be equal, remember), the trace will stop at the first actor that BLOCK the trace (such actor have the specified channel response set to BLOCK), without completing the cast to the END location. So , by casting a trace box from A to B , if the trace find a BLOCK actor between the two point , this new point C will be the end of the cast, for a final cast from A to C. For using the multy trace cast to return all the actor in the specified area, the actors must set the channel responce to OVERLAP, in this way the trace will not stop to the first actor it finds.
I had the same problem but Nylira suggested the overlap / block solution. The length of the returned array was 1 in my situation too. I guess that if the trace find some overlapping actors before finding a blocking actor , the array length will be higher than 1.