Message: Error Building Site Model

Hi,

 I am sometimes getting an error message while modelling or as i am opening up my .gsf file that says "Error building Site Model".  The most obvious problem is that batter slopes no longer appear to be part of the model.

Anyone else have this problem often and no why it is?

On some projects I seem to be constantly remodelling things due to getting this error message.

Any help  would be greatley appreciated.

Cheers,

 Chris