The Donnager had one frigate, the Tachi, available. The Wiki says that there were more ships. After the battle in episode 4 of season 1 got serious, why weren't these smaller vessels deployed to help out with the battle? Or were they?
I don't have the episode in front of me right now, but here's an answer from the books.
The Donnager was caught off guard. When the Martians intercepted the Knight, with our heroes in it, they assumed the ships following her were chasing it. However, they were surprised when the ships took on the Donnager itself, and succeeded.
The Martians could have deployed the frigates before the mystery ships came close. Hubris made them believe that wouldn't happen, but they were wrong.
First, the mysterious ships outmaneuvered or destroyed the Donnager's torpedoes. Then, they managed to sneak some torpedoes of their own past the Donnager's point defense cannons. This made the ships able to get close enough for close-quarters battle, at which point it was probably too late to get the frigates into the fight.
Then, the ships surprisingly fired rail gun slugs at the huge ship. (The frigates probably wouldn't have been able to do anything the Donnager hadn't already tried, except that they were faster and couldn't be hit by the rail guns.) And then, of course, the ships actually boarded the Donnager, at which point the frigates were useless – and the boarders placed half a dozen people in the hangar bay anyway.