The major reason why Russian invasion is failing is because of the lack of artillery support. The whole Russian land nased operations revolves around attrition to enemy by arty and preservation of own forces.
Failing? It’s only a mere 4 days. Let's break it down a little bit
USA took 300.000 soldiers to Iraq-- a country considerably smaller than Ukraine, and not nearly as densely populated. It took the US 3 weeks to take Iraq. Iraq was getting 0 outside support and had no intelligence capability nor was Iraq being fed intelligence around the clock.
Ukraine, by contrast, has full intel around the clock from the entire NATO, is armed and is still being armed by NATO, is very densely populated, much larger and Russia has only about 30-40k troops inside of Ukraine at the moment. Russia is moving quickly, while doing an outstanding job at avoiding civilian casualties in an invasion of a country this large and majority of Ukraine's forces using cities as strategic assets. From a military perspective, Russia is still employing a minimum of its capability in terms of manpower & especially, firepower. Russia is using very simple weaponry & used nothing it has developed in the last 20 years -- except Kalibr. No evidence of combat drones, loitering munitions, hypersonic missiles, EW, or widespread airstrikes. This is a fraction of what RUS uses in Syria Casualties, a realistic estimate which is still likely inflated if considered for Russian forces on their own -- is from the British MOD, which estimates 400 Russian losses. This is likely accurate IF combined with forces from LPR and DPR.
The most contrasting thing is this: the US was not worried about civilian casualties, they were simply collateral-- and it still took them 3 weeks. Casulties: This is from 2008, number is much higher these days Russia's invasion of Ukraine has only just started, and that is in context of timeline -- and also capability employed. Russia is moving much faster through densely populated land, with 7x less manpower being used.
And yet we are failing?