Russia is so overrated at fighting.
The Soviet Union gets so much credit for defeating Germany in WW2 but they had a lot of help.  
1.  Germany moved 5th Panzer to Italy instead of Stalingrad due to Operation Torch and Hitler's desire to keep North Africa.  This contributed to Germany not having any German divisions to relieve Stalingrad
2.  At Kursk, USSR received tons of intel from the British that basically told them where Germany was attacking, with what they were attacking, and most importantly allowed Russia to take out most of the Luftwaffe before the battle ruining the pivotal offensive.  USSR was also heavily supplied by USA equipment via lend-lease for the battle helping their own offensives.  Even Stalin admitted that without US Trucks and AAs, the Soviet Offensives would NOT have been possible
3.  Germany became heavily overextended fighting in Italy, eventually France, and Eastern Front contributing to more and more USSR victories
Russia has struggled heavily in offensive wars (Crimean War, Russo-Japanese War, and even WW1 show that).  Russia's best victories have come in situations where they led the enemy into their own land and used terrain to beat their enemy (Great Northern War, Napoleonic Wars, WW2).  The only true offensive war that saw Russia perform very well was the Seven Years War against Prussia and Prussia was also having to fight Austria, France, and Sweden in that war.  
Russia got Poland thanks to a partitioned that saw Austria and Prussia working with them.  They got Ukraine partially because Ukrainians had a choice between the Muslim Ottomans or Christian Orthodox Russians (seems like a no brainer figuring in that Ukrainians were Christian Orthodox and the Ottomans were notorious for their brutality and murder of Christians).  No one really lived in Siberia so they basically beat up technologically inferior nomadic tribes such as Kazan, Uzbeks, and a very, very weakened Mongolian Empire.   
Russia did have some nice wins in the 1700s against the Ottomans and Sweden.  Interestingly, the era that saw the best performance for the Russian Army was under two female rulers Czar Elizabeth (who doesn't get enough credit as a great leader; it was probably because she chose the moron Peter the III to replace her instead of Catherine) and Czar Catherine the Great. 
In summary, despite claiming a lot of land, Russia has been one of the weakest military nations historically.  A lot of their wins have been using defensive tactics so it makes sense they are struggling in a major offensive campaign.  They really haven't had a great offensive campaign in their history.  Part of the problem always relates to corruption in their army and poor logistics which is what is hurting them now.