Not only is momentum to be considered, but what type of broadhead being used. If it was a mechanical then it's common knowledge that a certain amount of he KE and momentum is used to penetrate the hide and deploy the blades. Depending on the draw weight of the bow there may have been insufficient energy left to get proper penetration when bone is encountered.

I have this problem in that due to limited physical abilities my setups usually produce less than 40# of KE. Because of this I have to be very selective with my shots and most often get complete pass throughs. I also use a replaceable blade head with a narrow 1 1/8" cut.