Mr. Toms solution will absolutely solve your problem if the existing vents are in the wrong place.
But what is usually happening with this "phenomenon" is the issue will occur on the top/sprue-side foot...because...let's look inside the mold...the hot/thin plastic is coming from the leg then trying to fill the widening cavity that is the foot. To fill this top foot correctly, the plastic has to enter the foot-cavity, spread and push air at the same time, while NOT "falling" into the foot-cavity faster than the plastic is being fed and NOT faster than the air is being pushed out completely. Cooler/thicker plastic is better able to fill without falling and plugging the vent before all the air has escaped. Every time I have the problem, the void is on the "round" of the foot deep inside the cavity where only drilling a hole could vent and help the issue.