It was all fine before but suddenly now, when i load an alpha channel as selection, only the super white areas get picked up and not the grey areas. Not sure what I changed but it is doing it for all the channels.
Can you show a screen shot?
The marching ants mark 50% gray, so this is entirely as it should be. The rest is below 50%.
Note that this doesn't mean it's not selected. A luminosity selection copies the full tonal scale.
But also note that any on-screen single channel representation is interpreted according to your working gray. So with a different gamma/dot gain it could well be that the dark grays will tip over the 50% line.