I just tried it with TRC and a couple others, and had it grabbed each time with no slippage, I even hung on it and it grabbed after descending a little. For some reason the wraps aren’t getting the load after they have been compressed for you. Did you happen to try hand tending it and then weighting it, wonder if it would grab after?
First suggestion is to add another wrap, but when it’s somewhat borderline on functioning, that usually means too much friction. Maybe try twisting both ends about 180 degrees after they pass through the bridge coming from the top, and then tie to carabiner. We are all in trouble if it takes an expert to operate hitches!