By the way, the reason why Ryuken did not practice as a Quincy for such a long time was because of his relationship with his father. If you followed the little backstory of Ryuken we got just before the Hueco Mundo arc, you'd know that Ryuken told Ishida that he thought his father (Ishida's grandfather) was wasting time saving spirits from hollows when he (Ryuken) could do so much more in the real (non-spirit) world, maybe even preventing people from dying in the first place and hence also the possibility of them being eaten by hollows or becoming one. This would imply that Ryuken chose to stop being a Quincy out of choice (and it wouldn't surprise me that the death of a loved one is involved). Why he returned is a different question, but as Squad said, it seems fishy that he did when Ishida was in the most need of returning his. My personal belief is that Urahara had something to do with it. It is also important to note that we have not seen Ryuken kill any other hollows more than that arrancar that was chasing Ishida, meaning we have no absolute proof whether Ryuken is actually practicing again. Hopefully we get some more of Ryuken's backstory too now that Kubo promised us Isshin's.