I've only formally signed up with one agency, at the moment & so far I don't have too many complaints, they have been very helpful with offering advice on my CV, they conducted a mock interview & so far have got me one real one.
On the other hand via a separate 'CV match' service I've had my details sent off to two other agencies & haven't heard a dickey bird..
The main difference is that the 'good' one has specialist divisions for various employment sectors at a reasonably 'professional' level & the clients can make sure they are dealing with people who know the market, wheras the others are general purpose, dealing with everything from fish finger packers to lorry drivers.
So, based on no more than recent experience, Arthur, I'd see if there are any agencies specialising in your preferred employment sector & try to target your applications more, rather than a using a 'scattergun' approach ??
Good luck getting another job, btw