When the Warsaw Ghetto Uprising started (April 1943), the Red Army was busy routing the vonPaulus's army near Stalingrad, so there was not much it could do to aid the insurgents.
By contrast the Warsaw Uprising started when the Red Army was across the Vistula river (August 1944). Historians debate whetethere they could help the insurgents, but there is considerable evidence that they could not. The question is, however, whether they should. The Uprising was started by the Polish bourgeois government-in-exile in London to thwart the ascent to power of the pro-Soviet "Lublin government." It was a last ditch gambit of Polish nationalists launched against the explict warnings from both Churchill and Roosevelt that the allies would not aid the effort. There is considerbale evidence that the Polish "London government" withheld that information from the Resistance commanders in Warsaw, who believed that that would get help as soon as fighting would start. That in my view, amounts to treason - far greater than Vichy - because of heravy loss of civilian life.
It is no wonder that Stalin did not want to aid the insurgents. Why should he aid an effort directly aimed to oppose his policies? It would be really stupid if he did.
wojtek