To answer this, just look at two things. First, what should it look like if the Ministry of Plenty did a good job? Second, what does Parsons ask Winston?
If the Ministry of Plenty is truly doing a good job, you would think that the people would be well off. They should be well fed and they should have (at the very least) all the essentials of life.
But look at what Parsons asks Winston. He asks Winston whether he has any spare razor blades he could give Parsons. This means that there is a razor blade shortage -- Winston says he's been using his for 6 weeks and no one thinks that's strange.
To me, this is ironic because Parsons somehow believes the Ministry is doing a great job when, at the same time, he can't even get something as basic as a razor blade. That's ironic because the two ideas (Ministry doing great; he can't get a razor blade) are so contradictory.