The appliance can go from Full Access to Read only status for a few reasons.
Ones that may be applicable in your scenario would be - Disk Capacity reaching high threshold or being in the Maintenance window.
You can check the capacity of your VDP appliance on the Reports tab.
You can also SSH / Putty / use the console to log on to the appliance and check the /data01/cur/err.log. This would show if there were any disk errors that might be causing the appliance to switch to read only state.