Premiere Pro: can't switch on A1 button

Hello from Switzerland, I am editing a wedding video. I used a mix of footage (mirrorless and DSRL). When I started editing all my clips had visible audio. Now when I drop certain clips on the timeline some don't have audio, others do. What I noticed is that the first A1 button is not blue (the one completely to the left), whereas the V1 is activated.  But for some reason I can't click on it. 

What I also noticed that I can only drag the video (the audio button is even not visible), but the clips do have audio.

I am lost, has anyone experienced this before?

Thanks for your reaction and have a great day, Erna