People shouldn't expect wonders. Russia had a lot of time to fortify like crazy. Rows after rows with thousands of mines and trenches and artillery aiming at the positions where attacks are most likely. And if Russian soldiers have to give up a position they have a lot of back-up positions with new trenches and so on... that's all something they didn't have when Ukraine stormed Russian troops around Kherson, Kharkiv, Izium and other important cities/areas.
It would be also crazy to throw everything at the fortified position now without trying to check which ways could be the "easiest". Too bad the West didn't act faster with the necessary equipment and the weather was also shitty especially for those heavy tanks. That definitely helped Russia to make it much harder for Ukraine and will cost more lives as it was necessary. But they will still succeed over the long run.