1973 Cent - Missing Bud..?

Quite common on the '73. You find them like that on the '74 as well. You can get full buds, partial buds, or like yours, no buds at all. I thought it was greased-filled dies or a weak strike when I found my first one or two, but now I've found so many of all stages of bud size that I'm not sure. I think a weak or low pressure strike might be the answer because it's often found with a similar weak strike on the obverse side in the center of the coin -- around the queen's ear.