Stealth not only involves snazzy wing and tail angles and recessed jets in the body, but radar absorbent skin and low emission electronics on the aircraft.
Russia is having problems affording the PAK FA project. Russia's invasion of Crimea and covert action program in eastern Ukraine, has led to Western economic sanctions. The flow of Western dual-use technology helpful to the PAK FA project has also slowed. Also low oil prices have damaged the oil-export-dependent Russian economy - leading to a decline in the Russian GDP of 3%.
Due to low revenue and high PAK FA prices Russia only plans to buy 12 PAK FAs in 2020 instead of the planned 60. By 2020 the US may have 500 stealth fighters (the F-22s and new F-35s) leaving Russia way behind. Much more detail here.