I think that there was poor after sales support especially when the aircraft suffered technical problems. The provider of the needed spare parts did not provide them in a quick enough timescale. I believe the leasing contracts that the Superjets had did not permit operators to have a stock of spare parts (this is rumour I have heard and I can’t confirm it). However I believe Aeroflot have a different deal and are able to carry a stock of spare parts, thus downtime is a lot less in the event of snags for Aeroflot
It´s not that you are not allowed to get your own spare parts pool, quite the contrary, for example the solution to the endemic Sam146 problems from Powerjet and SCAC was to tell their customers "shove it, if your engines are failing then get some spare engines by yourselves". Powerjet would take the engine and repair it as part of the warranty, that is if and when Safran would want to provide for example the combustion chamber replacement, but Powerjet or Safran would not overtake the costs of replacing the engine, the revenue loss the downtime this represents for the airlines, etc. This is the big difference for example to what is going on with the equally problematic P&W GTF, Sam146 customers are being screwed by SCAC/Powerjet and Safran is screwing SCAC simultaneously
The other main issue is getting spare parts from certified third parties, as part of their projected income both SCAC and SJI included selling spare parts all by themselves to SSJ operators, instead of creating 3rd parties that could handle this in a better way and have a much better distribution network. The end result, there is not enough availability of even ancillary components such as pumps, sensors or other small parts that could ground any SSJ in operation in a remote location in Europe or Mexico for example. Supposedly, there are steps being taken to correct this but of course solving this type of problem won't happen overnight. Even SN´s CEO said that the SSJ is not more unreliable per se than other aircraft types, the issue came when you needed to fix some small detail that wouldn´t normally incur in grounding any other type, but that would result in a grounded SSJ. Let´s hope lessons have been learned, specially for the MC-21 which in theory offers a state of the art and very competitive product against the Western duopoly, or what COMAC is offering with the C919.