Most modern submarines are built with air-independent propulsion (AIP), but Russia still has not been able to make it work. This means that Russia still builds Kilo-class submarines with a propulsion system that is technologically outdated.
A submarine without AIP needs to recharge the electrical batteries ever so often through a snorkel operation that puts the submarine in a vulnerable position where it can easily be discovered. Therefore, when Russia started to develop the Lada-class submarine in 1997 as a successor to the Kilo-class, AIP was the primary feature.
The Lada-class may overall be considered a fiasco, as the AIP system still doesn’t work. Only one Lada-class submarine has been built so far, and two more are underway, but they don’t have AIP which was supposed to be the defining feature of the class. And as is stands now, it seems a long way out before Russia will have AIP submarines that are not nuclear.
Sébastien Robin has this explanation in The National Interest about Russia’s troubles with AIP and the production of the Lada-class:
[In] 2013, Itar-Tass announced that work on the Lada class had resumed. However, actions speak louder than words. In the same time period, the Russian Ministry of Defense ordered six additional Improved Kilo Project 636.3 boats to serve in its Pacific fleet. Most experts agree that Russia simply wasn’t able to develop an effective AIP propulsion system, in part due to a pervasive lack of funding and a tendency to promise big new projects that frequently fail to materialize.
Though Russian officials have occasionally talked up the benefits of AIP-powered submarines, necessary research and development funding has been concentrated on two nuclear-powered submarine projects, the Yasen-class attack submarine and the Borei ballistic-missile submarine.
That is certainly a plausible explanation. The whole article is very well written.