As a newbie to the iMPC pro I'm having the same issue with the timeline view, but think I've worked out the problem.
I'm guessing that the timeline only show information recorded on ONE TRACK. So, if like me, you're layering across multiple tracks (as I would, working on my 2KXL) you will only see information for one track only.
I guess Retronyms are under the impression that users will record several elements onto one track (snare kick hats) where as most experienced composers will separate their sounds, so they have more control when arranging a sequence or song.
I'll check this out with Retronyms, and if it is the case, they will need to think about an upgrade to the app, ( or refund)
Uncle