The first time we compile a program including copybooks on mainframe, compilation is successful.  The second time we compile the same program, the compilation gives an error that the copybook cannot be found.

This problem can occur if the mainframe datasets/PDS are not suffixed with CPY. To resolve this there is a dataset override in the Server Dataset Configuration.

To set this in Mainframe Express 3.1:

Select OPTIONS > SERVER ACCESS CONFIGURATION > SORCE CONNECT TAB > CONFIGURE YOUR DATASETS

You can then add your PDS NAMES and CPY as the Extension.  This will now pick up the mainframe copybooks on subsequent compiles with Caching switched on.