Any recruiters here who can chime in on this?

Some of the more archaic ATS's make it really difficult, so chances are they're working on one of those. Most should make it very easy to see, which is why I advise against applying to too many roles in one company - choose one or two max, and really go for them. Another reason could be if they don't have the proper permissions to see past roles from other teams. Some companies are very siloed, and recruiters from different parts of the business wouldn't even know each other. This of course depends on the size of the recruiting team and the organization's culture of communication (or lack thereof). I hope this helps!

It just depends on the system. Some ATS software just really sucks and doesn't make it easy to search. That said, it's only a small few that I'd say are that difficult (looking at you, Workday ATS!) and in most cases it's not difficult.