My equipment isn't getting me enough monster heads

I'm playing the Hexxit modpack and have made a couple of Tinker's Construct Cleavers, which are supposed to have a Beheading trait by default, but they have thus far given me no increased rate of mob head drops. I then explicitly added the Beheading trait to one of them and ran around killing many things, but still my trophy wall of heads remains without any new tenants.

I'm quite disappointed at my lack of decapitation capabilities. Is this a known bug, a configuration issue, or some third thing?

I do not know the current version of Tinkers your are running but here maybe a anwser to your problem.

According to the Wiki:

Obsidian + Ender Pearl

Adds Beheading (a chance for mobs to drop their head) to a weapon, single use. Stacks with previous levels of Beheading.

Lapis Lazuli

Adds Fortune/Looting, requires 100 lapis lazuli for level I, 300 for level II and 450 for level III. Not compatible with Silky Jewel.

Be sure that you have the most amount of chance in order for skulls. Remeber: skulls can only come from vanilla mobs (ex: zombie, creeper, wither skeletons) unless added by another mod. Hope this helps.