How on earth to progress on this game? stuck at act 6

[Rambo Redfield]

I know you have to complete the challenge missions the the thing is ever since ive beaten first boss and ran thro next 3 acts challenge acts do not appear.

[Leonard]

You haven't completed City Escape and Seaside Hill yet.
You only need to do one challenge mission from every level to get the key.

There is a modern and a classic version of every level. You need to complete both.
Then you have to complete any of the missions from each level (those doors above the level gates) and collect the key.
When you have all 3 keys you can go to the next boss which unlocks the next set of levels.
There are also the 3 rival battles

[Rambo Redfield]

Thanks Leonard! i didnt completed Classic Version, it has been ages since i played this game and continued since the last save, totaly forgot SG has 2 playable versions of sonic character :)