I have applied in the past for a number of posts through agencies and if you really push them they will give you a little more information, namely that they are working on behalf of PremiAir or PAS. PremiAir and PAS of recent have gone to using agencies, god knows why as it costs them a small fortune.
The question begs if PremiAir or PAS are unable to locate suitable applicants by advertising direct with the likes of Flight International, why do they feel the agancies are going to have any more luck.
At least if you go direct to PAS or PremiAir your more likely to get at least a response.
I hasten to add not all agencies are working on behalf of the afore mentioned.
Best of luck with your endeavours.