NLA strip sometimes not working

I am having a problem with the NLA editor. Don’t know if it’s a bug or something I’m doing wrong. I’ll try to explain it:
I’m using an empty to parent the armature of a fish and move it across the screen. While making changes in the graph editor, I notice there’s a problem; changes don’t seem to be staying. I go back to the dope sheet and ‘push it down’ anyway. Then I look at the new strip in the NLA editor. I notice that the new strip is translucent-brown and there’s no padlock symbol for that NlaTrack.001.

I’ll attach a screen-shot.

I can delete the old strip (yellow one), and move the new brown one down from NLA track.001 to NLA track. There it turns yellow and looks ok. I find the changes I made HAVE been noted. But on repeating this, I sometimes find the strip no longer works - even if it hasn’t shown as brown (and the nla track DOES have the padlock symbol).

Does anyone know why this might be?