Weird Matrix behavior

I'm reworking a song with Matrix View: adding loops from the song to cells and also adding new material to the cells. I'm using 4 columns and 4 rows and all is working well.
 
With MIDI learn I programmed the triggerpads of my Roland A-500Pro keyboard to trigger the columns. This didn't work well: columns are triggered but also other things, so I cleared MIDI learn. Now the cells nor the columns play when clicked. They flash, although Follow transport is off. When I add a new loop to an unused cell in an unused column, everything works fine.
 
Any help is welcome!