Russia has difficulty mobilizing replacement troops

 ISW:

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Russian forces continue to face difficulties quickly mobilizing replacements for combat losses but are continuing efforts to leverage conscripts and international troops. Ukrainian intelligence reported that Russia has been forced to suspend the mobilization of certain categories of workers in the Donetsk and Luhansk People’s Republics (DNR and LNR) to maintain local governance but continues to conscript some DNR/LNR residents to replace Russian losses.[2] The Ukrainian General Staff reported on March 15 that Russia has established mercenary training centers in Smolensk, Russia, and Homel and Bobruisk, Belarus.[3]

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Main effort—Kyiv axis: Russian operations on the Kyiv axis are aimed at encircling the city from the northwest, west, and east.

Russian forces conducted limited attacks against several towns close to the western bank of the Dnipro River on March 15. Russian forces did not conduct offensive operations east of Kyiv and continued to prioritize improving logistics and reinforcing combat units. Russian forces are unlikely to launch offensive operations to encircle Kyiv larger than the scattered Russian attacks observed northwest of Kyiv targeting Irpin on March 14 and Guta-Mezhyhirska on March 15 within the coming week but may launch further tactical attacks.

Subordinate main effort along the west bank of the Dnipro        

Russian forces conducted several unsuccessful limited attacks toward northwestern Kyiv on March 15. Ukrainian forces repulsed Russian attacks on Guta-Mezhyhirska and Lyutyszh, directly on the western bank of the Dnipro River, and on Makariv, directly west of Kyiv.[4] The Russian Ministry of Defense falsely claimed Russian paratroopers captured Guta-Mezhyhirska late on March 14 and seized a cache of “Western” weapons from “nationalists and foreign mercenaries.”[5] The Ukrainian General Staff additionally reported ongoing fighting along the length of the Russian effort to encircle Kyiv from the west on March 15, but ISW assesses that Russian forces have not conducted any major attacks.[6] Russian forces continued to deploy replacements and reinforcements to the northern Kyiv axis

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Rusia appears to be unable so far to mass enough troops to break through the Ukraine defenses in many locations.  By spreading its troops too thin, they do not have the combat force for the breakthroughs they are looking for.  They still seem to also be struggling with their logistics still.