Ok - got to the bottom of this
There is no limits for parameters. But I think because this is an old project - some of the controls ended up with duplicate control ids. A while back I added code to prevent this from happening - but I think because your project is quite old it predates that fix.
So - if you download the latest version. Find the control that won't let you add - in normal (not snapshot learn mode) - right-click control - Tools->Regenerate ID.
Then try relearn the control.
I worked this out mainly because your horizontal slider send that won't add - actually deselected another control from the snapshots. So you may wish to go back to an earlier version if possible before you subsetted out the page snapshots to ensure they aren't missing parameter which was being deselected when trying to select the missing parameter.
Best not to do this (Regenerate ID) for any other reason as in other cases it will leave your snapshots messed up slightly - but if the control hasn't been added to a snapshot (because it has duplicate ID) - it should be fine. Remember to back up just to be sure
So this regen id tool should only be used to fix old projects - no reason to use it otherwise