Personally, I wouldn't recommend a expandable with anything less than 60# bow wt.
I recommend a cut on contact broadhead, and perfer the Magnus 4 blade Stinger, for your set up.
We are using the Stingers, (Me, Wife and 9 yr old Son)we have had no problems with penatration.

I'd be inclinded to believe that the arrow, after impact,is loosinga lot of energy and just can't "punch" through with the mechanicals.
Try a cut on contact and let us knowhow that works!