Audio plugins not working on mono vocal track

Hi!

I have a strange problem with a mono vocal track. All the plug ins I'm using on this track (and on this track only, on all the other tracks I do not have this problem) are greyed out. The track is "on" and armed and I hear Audio. But the plugins won't open or work. When I put the track in Stereo, all of them miraculously work. But it's a mono track with a mono recording, so I would like to have it in mono... Super weird.

Any Ideas?

Thanks, Dave