Forgot to say, the images after editing have been saved and closed down, so nothing open in Photoshop when trying to open new images for editing. (It's a brand new Macbbok pro so won't be upgrading for a while!) I don't have the same problem when using iMac which has only 4 gb more Ram & can open 20+ in one go
In that case, you could try allocating more memory to Photoshop. Photoshop > Preferences > Scratch Disks.