Sure give them better defensive support but that isn't mutually exclusive from teaching them good defensive play. Doing both will lead to superior results then just addressing one issue.
The point is, start with the major issues and work out the rest of the kinks when the big issues are addressed.
What appears to be no defensive progress with certain forwards, might simply be that they are not playing under decent conditions due to changes in the roster. Our D has been horrid lately - I'm not sure that is the best time to evaluate our forwards defensive game.
We're using stats to confirm a player is good or bad in certain parts of the game when underlying major factors are impacting our ability to make these assumptions. The only assumption I would take away from poor defensive metrics at this point, is that our Blue line is bad - and we know that already.
Fix the D first and then begin measuring our forwards under conditions that will allow for a fair evaluation. At this point, these players have 2 or 3 years more experience as players - maybe they are better defensively than they were but we won't know until we've leveled the playing field.
Of course we want our players to be their best at both ends - but hold off on determining how much work they may need until we've fixed the issue that is having a major impact on their ability to play a complete game.