Are my parents suffering in Heaven (or Hell) because my brothers and I don't cook Koliva for them?
We do memorials for people for two reasons.
1) To pray for them, thereby expressing our love for them in prayer, as commanded by God, Who desires us to pray for all.
2) To remind ourselves of the two realities that give us Christians hope, even in the face of death: a) That our loved ones who have died in Christ remain alive in Him, praying for us in Paradise. b) That they will rise at Christ's call at the Resurrection, their mortal bodies re-united with their souls, that the bodies we sowed in corruption at their burials will be raised in incorruption and glory, as witnessed by the use of the grain in the koliva, referring to the Lord's words in John 12:24: "Truly, truly, I say to you, unless a grain of wheat falls into the earth and dies, it remains alone; but if it dies, it bears much fruit."
So no, I think it is safe to say that your parents are not suffering because you are not cooking Koliva for them. They may perhaps be grieved that you do not remember them in prayer, but they do not suffer.