I made it work by adding the repositories to the settings.xml
like this:
<repositories>
<repository>
<id>mycompany-releases</id>
<url>https://servername/nexus/content/repositories/releases/</url>
<releases><enabled>true</enabled></releases>
<snapshots><enabled>false</enabled></snapshots>
</repository>
<repository>
<id>mycompany-snapshots</id>
<url>https://servername/nexus/content/repositories/snapshots/</url>
<releases><enabled>false</enabled></releases>
<snapshots><enabled>true</enabled></snapshots>
</repository>
</repositories>
Then, the SNAPSHOT jar files were downloaded just fine. I suspect that when Maven knows it deals with a snapshot repo, it tries both with and without uniqueVersion (see Duncan Jones' answer).
Note that in our case these blocks had to be duplicated as pluginRepositories
because we have custom Maven plugins.