Facts we should consider
-Isshin revealed to Ichigo of the existence of the final Getsuga
-Using the final Getsuga will cause the loss of one's shinigami powers
-Ryūken Ishida father of Uryū Ishida once stated, 'So your shinigami powers have returned' to Isshin (paraphrased)
Having analysed these three points, it would SUGGEST (requiring further validation) that Isshin had once used the final getsuga, lost his shinigami powers for x years and recovered them somehow...
For further arcs of bleach to continue, Ichigo MUST regain his shinigami powers if he does in fact loses them in the up coming weeks. In parallel, Isshin was able to regain his powers in an analogous fashion IF in fact he had lost them using the final getsuga.
Isshin lost his shinigami powers in a totally different way, leaving my hypothesis and this post completely redundant.
What do you think?