An error message is generated when trying to select more than one element: "H&H-WaterGEMS has stopped working"
First, close the program and browse to where the model files are saved. Select just the .wtg and .wtg.sqlite files and move them to a new folder location. Next, open the model files from the new location.
If that doesn't resolve the issue, go to File > Import > WaterGEMS database and select the .wtg.sqlite file associated with the model. That will import just the database file into a new file. And color coding or annotations will need to be recreated if the database is imported into a new file.