Mattias Ohlund's PPG dropped for three consecutive seasons in the SEL following his draft. RJ Umberger's scoring dropped slightly in his D+1. Daniel Sedin's scoring plateaued in his D+1 while Henrik's increased. Jannik Hansen remained in a low-level league in his D+1 and was at a PPG pace in junior in his D+2. Ben Hutton's scoring dropped in Junior A in his D+1. Mason Raymond was so-so in the NCAA in his D+1. It happens quite often. I don't think Melvin's overall impulse is incorrect, but it needs to be fleshed out considerably.
Ohlund stepped up from tier 3 (The same level from which Alex Edler was drafted that articles about him used to refer to as being nearly a beer league) in his draft year to the elite league in his D+1 and then won Junior aged player of the year in his D+2 while winning an SHL championship and being named top defenseman at the WJHC. What are you talking about?
Umberger stayed steady despite his team having their second highest scorer (who had posted similar numbers the year before Umberger arrived as well) graduate out. Didn't blow up, but certainly had a reasonable year.
Daniel Sedin led the SHL playoffs in goals in his D+1 and in the regular season was second on his team in scoring behind his brother. Was third on the Men's WC team in points. Not really all that disappointing season, but probably didn't shoot out the lights either. They'd already set an extremely high level of play, though, and I don't know what you could really expect.
Hansen significantly improved his scoring in the Danish league, prompting Portland to use an import slot on him. Can't be considered all that disappointing considering he was drafted out of Denmark in a round that doesn't even exist anymore.
Hutton's PPG improved decently in his D+1 if you're talking about his draft year by age, so you're wrong there. If you mean the year after he was drafted by the Canucks, he wasn't playing Junior A, so you're wrong there as well. He went to the NCAA and led his team's D in scoring as a rookie.
Mason Raymond was second on his team in scoring as a true freshman, and made the WCHA all-rookie team. That's not really a "so-so" D+1 for a player taken outside the first round.