Hi,
This can happen for a number of reasons. Sometimes freeview can switch the transmitter or your aerial may have been moved in any high wind etc.
If you have a freeview TV, you can also connect your aerial direct to the TV and also try a retune and see if this has the same issues. If so, then this is solely a freeview issue. You can check your aerial visually to see if it is facing the same direction as others. If freeview are working on any transmitters in the area, then you would have to wait for them to resolve.
Thanks
Karl.