Inspect the bolt. On the newer style, as pictured, there are two different sizes of o-rings to seal the bolt. One the earlier style, there is only one o-ring.

On this bolt, the o-rings are old, brittle, and flattened. They need to be replaced, there are o-rings in most metric o-ring kits that will fit.

On the early style housing there is also a copper crush washer that goes under the head of the bolt, in place of the second o-ring. This should be replaced with every filter change.

See the new o-rings installed here.