Russian reinforcements failed to breakthrough

 ISW:

Russian forces continued small-scale, tactical attacks on the Izyum and Severodonetsk axes; additional reinforcements to date have not enabled any breakthroughs of Ukrainian defenses. Russian forces continue to deploy reinforcements to eastern Ukraine but show no indication of taking an operational pause. The Russian military appears to be carrying out an approach in eastern Ukrainian similar to its failed efforts north of Kyiv in early March—continuing to funnel small groups of forces into unsuccessful attacks against Ukrainian defensive positions without taking the operational pause that is likely necessary to prepare for a more successful offensive campaign. Russian forces continue to grind down Ukrainian defenders in Mariupol, though ISW cannot currently assess how long these forces will hold out and their current supply status.

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Ukrainian military intelligence reported on April 15 that sanctions are increasingly disrupting Russia’s ability to replace equipment losses.[1] The GUR stated that Uralvagonzavod, the largest tank manufacturer in the world and producer of Russia’s T-72 main battle tank, is facing rising interest rates on loans, rising prices on materials and components (including armored steel), and shortages of imported components. The GUR said that Russian import substitution efforts are failing to maintain existing contracts.

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Russian forces continued unsuccessful daily attacks against Rubizhne, Popasna, and Marinka and heavy shelling along the line of contact in eastern Ukraine but did not make any territorial advances on April 15.[6] Ukraine’s Defense Ministry stated on April 15 that Russia's 58th and 8th Combined Arms Armies are the primary Russian forces in Donbas, in addition to the 1st and 2nd Army Corps of the DNR and LNR, respectively.[7] Russian forces likely require a true operational pause to cohere the forces that could enable them to advance through frontal assaults in eastern Ukraine.

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I get the impression that the Russians fear doing the kind of operational pause that would be necessary for them to achieve their stated objectives.  They may fear that their current morale problems would get worse if the troops were not operational.  The small-scale attacks they are engaged in have not moved the needle on their main objectives.