When their push is slow with the current troops they have. For context, Ukrainian troops outnumber the numbers of Russian troops massed at the border before the war?
Because most personnel of any army looks like this:


Non-combat tasks include mowing the grass

Fewer than one in ten soldiers in any army are combat personnel. In modern armies the ratio is closer to one in fifteen. The rest are mechanics, cooks, laundrymen, medics, truck drivers, security personnel, firefighters and more. You have judges, lawyers, criminal investigators, microbiologists, psychotherapists (one word), civil engineers, programmers, PR personnel and so on and on and on.

Pick a job in a civilian field. Almost all of them have a military counterpart.

This is no different in Russian army. In truth, the 15% or so of the entire Russian army deployed to Ukraine represents approximately 85% of their combat personnel. Only a few small formations are still deployed elsewhere, nearly everyone who is expected to go to combat went to Ukraine.

Of course in a desperate WW2-style situation you will use everyone and their dog. Ukraine is certainly sending their cooks and truck drivers to war with rifles and limited training right now, but these men fight for the very survival of their nation. They will be as effective as soldiers with limited training can be.

Those same formations would only eat up the logistical apparatus for the invading Russian horde. Russians have enough problems with those as it is. They started running out of fuel and food on day two, which is a national disgrace.

It is ONLY a fool, a very big one, that will deploy even half of his troops to fight one enemy when he has dozens of other enemies. Where is the sense in that?
“If he sends reinforcements everywhere, he will everywhere be weak.”
― Sun Tzu, The Art of War
Russia shouldn't because their advance is slowing down, deploy more troops. Most of their troops should remain in their motherland. The conscripts should be the ones to be deployed to Ukraine

This war just exposes how lacklustre and untactical the russian military is, they are just feared for the multitude of military gear and nuclear weapons they have, imagine going to fight the US or china with this kind of execution

NuclearReactor:
This war just exposes how lacklustre and untactical the russian military is, they are just feared for the multitude of military gear and nuclear weapons they have, imagine going to fight the US or china with this kind of execution

It actually took coalitions forces with the almighty USA, UK and co three weeks to take Baghdad. An Iraq without air defense and not as updated as Ukraine.

https://www.foxnews.com/politics/putin-maintains-95-percent-available-combat-power-ukraine-war.
Putin maintains 95% of available combat power in Ukraine presently. Intense fighting is going on. It is only myopic persons that would think putin is taking it lightly with Ukraine. It is only nuclear weapons that Russia has not used. Today, Russia dropped 1,000kg of bomb. No fatality but some injuries. What is helping Ukraine presently are presnce of air raid alerts and bomb shelters .

MangekyoAlt:
It is ONLY a fool, a very big one, that will deploy even half of his troops to fight one enemy when he has dozens of other enemies. Where is the sense in that?
“If he sends reinforcements everywhere, he will everywhere be weak.”
― Sun Tzu, The Art of War
Russia shouldn't because their advance is slowing down, deploy more troops. Most of their troops should remain in their motherland. The conscripts should be the ones to be deployed to Ukraine

Relax in military doctrine 5 to 10% of military personnels in direct fighting . Others are support staff.