You can't blame the writers for everything. Beltran and Montgomery got their episodes in which they could shine and Fortunate Son and horizon were great ... accept for Montgomery's performance. I don't wanna blame them, sometimes good actors don't match a role.
Beltran on the other hand never brought any interest to it. You could see how bored he was during playing and he later frankly admitted that he hated the show. Which is fine with me, it is after all just a job. But if you don't work hard you have no right to whine about not getting better scripts or more spotlight.
I can blame the writers for not writing characters in an interesting way. Yes, sometimes a great actor can make a dull character more interesting, but its easier the actor to make the character come alive if the character is written well.
Beltran and Montgomery have done well in other roles, so maybe if their characters were better written they would have turned in better performances.
Really the cast of characters in Voyager and Ent are not as interesting to me as the cast of characters from TOS, TNG and DS9.
If they make a new Star Trek show, they should have a plan or a purpose for every character, rather then just throwing characters in the show and hope everything works out. Having nothing characters like they have in the past would not work in today's TV market.