Primavera P6 wandering Baselines

Hello, I was just wondering if anyone could help me.  I am working on a number of programmes / schedules within Primavera P6.  All of which have baselines.  I have set the Project and Primary baselines within 'Assign Baselines', but every now and then when I return to the programme to run a filter or print, the baselines have re set back to >Current Schedule> for the Primary baseline or selected an alternative / old baseline for the project baseline.  Has anyone any idea why it does this or if there is something I am missing / doing wrong.  It doesn't do it every time I access the programme, sometimes it is as I left it.

Many thanks