Thanks for all of the suggestions.
I think I may be suffering from some kind of compiler bug. By process of elimination I worked out that there was something on the front panel/block diagram that was causing the problem.
If I create and call test.vi (instead of datalogger.vi) and just have a loop with delay and stop button, the main code will call and display the subvi as expected.
If I copy the block diagram code from datalogger.vi and paste it into test.vi then it stops working.
I have slowly been deleting section of code until it worked and there are two X-Y graphs that seem to be causing me a problem. If I delete one, it still does not work. If I reinstate that graph and delete the other, it does not work. If I delete both of them it works!!!
Very bizarre.
