You can turn the frontline to dust with artillery, but actually advancing when anything from a guy on a push bike to a modern tank is disabled by a quad-copter with an anti-tank explosive cable tied to it makes taking territory difficult.
Hence why Russia has advanced very little since the initial invasion, even with the number of men they are willing to send to their death.
Ukraine has a problem taking their territory back as well, but for different reasons. They don't want to/can't send so many to their death, and have to rely on a large coalition of allies giving them weapons -- who have their own things to worry about domestically, so half-arse or delay support.
Hence why Russia has advanced very little since the initial invasion, even with the number of men they are willing to send to their death.
Ukraine has a problem taking their territory back as well, but for different reasons. They don't want to/can't send so many to their death, and have to rely on a large coalition of allies giving them weapons -- who have their own things to worry about domestically, so half-arse or delay support.