I dont use the hobby brand for the bigger T-nuts. The hardware store T-nuts have never let me down. It sounds like the wood it really hard. In that case, I would use a smaller allen head bolt thru the T-nut then a washer and nut on the inside. Hold the nut with pliers and screw in the allen head bolt. The T-nut wont spin and should seat straight in. Remove the small bolt setup and install the proper bolt and seat it really good.
Edwin