The SVT40 was basically a very good rifle,its weakness was that it was too complicated to maintain for the average Soviet soldier,probably never got any training how to clean it including the gas system,especially since all ammo in those days was corrosive.
On this video it really shows how a case can get stuck in the chamber and how rusty the gas piston and cylinder had become.
Of course all gas operated guns would suffer likewise but the Germans really liked the SVT40 and used them a lot against their former owners.
They liked them so much that they copied the gas system in their G43/K43 rifles.
The one rifle that would not have suffered that much from neglect would have been the 1941 Johnson since it had no gas system,just a recoiling barrel that could be removed for cleaning in a matter of seconds,but it came on the scene too late and was never accepted by the US military.
As a matter of fact it may never have been produced if it wasn't for the large Dutch order for their colonial army in what is now Indonesia.