Sadly, for landlords, letting agents are sometimes (too often) unsatisfactory, and also sometimes for tenants. From what I have seen, they far too often simply exist to skim off a percentage of everything - rent and tasks such as arranging maintenance. I understand your dissatisfaction but you can only go against any stipulations in your agreement with your agent by invoking such things as notice to terminate the agreement and then following the terms of the agreement.
In my view, the best thing agents can claim is being able to locate tenants. However, I know of one case local to me where the agent took on a tenant who is a drug addict (poor/no vetting, I would say) and until recently was dealing but now appears to be selling herself to feed her habit. The result is pandemonium (noise, disturbance, violence, etc.) in the entire building owned by the landlord. Tenants are leaving - damage and income lost and meanwhile it takes months to evict the problem tenant, if that is even achievable (should be possible but involves court action).
While there must be good agents about, finding out whether one's own is any use can be a bit of a roulette.