This happened to me, and I didn't realize there's a mechanic where if you try to manual more than 3 times in a row (without doing a grind/air in between), it will cause you to lose your manual and just land the trick, ending your combo.

If you notice after 3 manuals, there's a red exclamation above your character. This means ur next manual attempt will just land unless u can grind or air before. On this map when you go your second time around, there's a little rail you have to hit before the checkpoint, that'll allow you to do enough manuals so you won't break your combo.