Adam, in addition to doing what others have mentioned (more bleeding,
checking mechanism) it is also possible that your drums are worn close to
allowable specs (diameter and allowed wear stamped on drum). When this
happens, the new shoes will not conform adequately to the curvature of the
drums.

Dennis Haynes made the suggestion (and I have done it successfully on my
van) to add a washer to the lower shoe mounts. Its obvious when you look at
it...what you want to do is shim the lower mounting "bits" out towards the
drum a tad. The lower mounting bits are a slide fit in the casting.

Alistair