Why are my homemade lobster rolls so much worse than those at the restaurants whose recipes I'm using?
Restaurants in general use WAY more salt than home cooks do.  I'd try increasing the salt and see what happens to the flavour.  The same is often true of butter.

Also, some restaurants that publish recipes, do not actually publish something that is faithful to what they serve.  I noticed it with a dish with which I was very familiar from a favourite restaurant. Certain ingredients were completely missing from the published recipe.

Edited to add:
The sweetness diminishes with storage time and the meat from the smaller claw is sweeter and more tender. (reference) It's also possible that a particular type of lobster or size of lobster is more or less sweet. Maybe you need to get multiple fresh lobsters and use only selected meat for your roll (and do something else with the leftovers)?  In On Food and Cooking, there is a suggestion that vanilla (normally used for sweet cooking) can be used when cooking things like lobster.  It doesn't directly state what the vanilla does, but it's possible that it might increase the taste of sweetness.  A bit of googling does turn up mention of lobster roll recipes with vanilla as an ingredient, so it might be worth experimenting.