The Russian defensive successes so far appear to be primarily related to not only their training and preparation & defensive fortifications but also combined arms and more effective organizations than last year- there are 3 divisions (127th, 19th, 42nd ) holding the defensive front besides infantry brigades, one special forces (22nd GRU) and a couple of separate volunteer battalions.
The Russian combat doctrine was designed primarily for defense against a NATO/US invasion, not offensive operations. The organic electronic warfare power of the brigades and the divisions are much greater than the battalion (company, battalion sized EW forces) and seems to have defeated Ukraine's drone army, or at least reduced its capabilities to a low level. It has also dramatically lowered the use ability for Ukraine/NATO to use HIMARS and Storm shadow missiles although some got through the EW zone. The Russian divisions and brigades also have much stronger Anti-air units supporting than the battalions of last year.
The Russians also moved forward their helicopter fleet which has a played a decisive role in knocking out AFVs with air-ground missiles and providing general support. The fighters of their Air forces are also in support.
The Ukrainian ammunition shortage has seriously effected their ability to provide important fire support to their attacking groups of infantry and AFVs, leading to poor results. The Russian side considers their casualties as being only light and 10 times smaller than their opponent.
Although the bulk of Russian front line fighters are mobilized reservists so far they have done their job, by repeatedly counterattacking and attempting to seal off breaches. They have the advantage of superior artillery and protection of defensive positions and extensive mining.
The Russians also have a self-propelled minelayer unit that can shoot and automatically generate minefields from long range. The infantry are equipped with the Fagot and other types of Anti-tank ATGM that can destroy or knock out all Western tanks in one shot from KMs away.
So far it is imperative for the Ukrainian-NATO offensive to break apart the Russian combined-arms team but so far they have not done it at all, and continue relentless but weak attacks by AFV and infantry groups. The much trumped western tanks and afvs have proven to be as low value as the Russian in the modern combat environment. I have no idea how Ukraine-nato have planned on confronting this problem outside of brute force and attacking with superior numbers.