Some kanji don't have a reading for one or the other. It may not exist. It's also possible that it hasn't been recorded (maybe by error). That's why I use dictionaries other than Renshuu alone. I have used tools about words/kanji that would go a bit beyond the scope of what Renshuu has to offer.
Also, if you used the options that restrict which reasing you see, they'll restrict the rare readings that would be featured otherwise. It depends which kanji you're talking about.
