Just a guess but, I would say the gear isn't installed on the shaft in the correct location or the surface in the block, (at the bottom of the distributor hole), where the gear should ride is worn beyond spec.
EDIT: I misunderstood and thought you meant the distributor gear.
The cam bolt is not strong enough or it is not torqued correctly or
high valve spring pressure calls for "double pinning" the camshaft.