Von denen hört man wenigstens ab und zu was …
We have just finished the first phase of a large project that established links from PubMed to most of our backfiles up to year 2001. The process relied on metadata matching that by itself was not precise, especially with historical data we are dealing with. That will also eventually plug in some of the missing links. There could be different reasons for missing links — it really depends on the journal in question and the publication year.
[Thanks to Dr. Lukas Mayerhoff]