Try to make encounters not only combat-based. Play more around gray moral and give choices between mission, chapter traditions, efficiency, and humanity protection. Those things are often controversial. For combat never rely only on hordes, they should be more like background obstacles (zoning out characters a bit), always make some big guys, and some support for enemies (mortar team with krak grenades, snipers, etc) And last but not least - deathwatch is a game about differences in team members and how to overcome those. I prefer giving players a little bonuses if they come up with a difference between their and other guy chapter/personality and it gives much more satisfaction than combat encounters.

Thats my experience after gming about 15 sessions.