Resistance generally is effective if there is;
1) Significant amounts of aid to the resistance.
2) The occupying army is not able to flood large amounts of troops into the area.
3) Political reasons exist which prevent the insurgency from being countered by various uses of excessive force.
4) The rebels have sanctuary in another nation.
5) A conventional army will appear to help and distracts the occupying nation. The rebels only need to support it.
In the case of an occupied Eastern Europe, most of these aren't in play. Aid to the region can only be so much, since outside of political reasons, it is just so huge that trying to get supplies deep into the interior would presumably be difficult. The Nazis can flood the region with huge amounts of troops. There is nothing to prevent them from deploying vast amounts of forces in. The rebels seem unlikely to get real sanctuary support from another nation, since the USSR would be terribly cowed by her defeat. And conventional armies cannot deploy for some time.
The rebels will be a nuisance, but sadly little more than that. Rebels succeeding and throwing off the authority of an occupying power is neither as often nor as succesful as commonly portrayed; the great "Wars of Resistance" that are generally pointed to, of Algeria, Iraq, Afghanistan (twice), and Vietnam (twice) all had many of the above situations working against the power fighting against the insurgency. And in some of these cases, the loss was not militarily the result of the nation fighting the insurgency. In Vietnam, the US beat the insurgency but withdrew and the loss happened to the conventional forces by the South Vietnamese government, in Algeria the French won the war but found it politically impossible to retain the region.