from what we know now, and what Urahara said. Isshin is Masaki's complete opposite. so they what exactly happened is that Isshin became a Positive because Masaki became a Negative, so when they joined their fates together it nullified/cancelled out all their powers so Masaki could live a normal but human life. That is how Isshin lost his powers. But because Masaki died (still under speculation that she did or didn't) it released him of his bonds and let him regain his powers back as there was no opposite to cancel out his powers.
Its like Math,
-1 + 1 = 0 Shinigami + Quincy = Cancel
or
1 + -1 = 0 Human + Hollow = Cancel
As for Ichigo being told he'd lose his powers by Young Hollow Zangetsu to the FGT, thats because of his unknown unmastered Quincy Powers. Look back at Uryuu, he was supposed to lose his Quincy powers permanently but didnt because the powers he lost were from a Quincy Combat Glove that amplifies powers and absorbs extra power from the surrounding area. He only lost the amplified power and some of his own. Ichigo on the other hand being of all 4 main souls is tied into an endless loop of power which essentially makes him a soul god, the powers do not cancel each other out like they do for Isshin and Masaki as they are 2 separate beings and Ichigo is one complete being. All Ichigos powers benefit him and each other.
