Visio 2000 – Shapes dropping to bottom of sheet automatically

Having created a number of charts on Visio 2000 SR1 (6.0.2072)) I find that when I save and re-open the file, all the shapes and connectors have dropped to the bottom of the sheet and become compressed. I have has to re-create the flowchart again and the second time there was no problem. I wonder has anybody out there had similar experiences and any ideas what might be the problem ?