It depends where you are in your acting career. Sometimes a shitty agent is better than no agent at all. It looks better on your IMDb and Actors Access, and this agent can have connections to casting directors.
However, if you have been with an agent for a year or two without getting any auditions or bookings, perhaps it’s time to move on and let go of them. I have a friend who had an agent like that, and he's not with them anymore.
Personal story: I had to let go of a certain agency not long ago, because I always got auditions from them as an extra in commercials, and never a lead. I got those auditions along with 50 other people from the same agency, not and that’s not exactly how it should work. Nor did I get any auditions for television shows or feature films. NOT ONE! Also, two people from the industry said that agency was no good, that they scammed people and never paid actors on time. RED FLAGS!
AGENTS, AGENCIES TO AVOID AT ALL COSTS:
If your agent takes more than 20% of your income- RUN! Agents take only 10-15%.
If your agent wants you to go to a specific photographer, to get headshots, specific teacher to learn from- RUN. Agents can recommend few people they know, but they don't MAKE you go to their friends who have a shitty camera and call themselves "Photographer".
If they want you to take certain classes and you have to pay the agency to represent you- GET OUT OF THERE! There are so many scammers in the world. Many of those agencies have open casting calls all year round, their ads pop on your Facebook stating that your child could be the next Disney star! BULL-FUCKING-SHIT. Don't get me wrong, kids get booked back and forth on cable network, but there really is no reason for your family to pay $1200+ a month to an agency who will keep fucking you over.
If the agent is exclusive and represents all of the Southeast market and isn't getting you any auditions in it, DROP THEM like it's hot. Exclusive agent means that you can't sign with anyone else in that region they're in. Non-exclusive agent is the one who allows you to have multiple agents in the same region.
To see other red flags, read the blog "Bad agents & toxic agencies".
One thing is that you absolutely have to do- never drop an agent before getting a new one. Get a new agent and then drop an old one.