Myself and the good lady have delivered tens of thousand's over the last year or so and it is a real pain. However, i see it as something you have to do your self to start off with.
You may have the most effective flyer in the business but if a company is sending them out for you, but not doing the job correctly i.e not delivering them you will never know.
You then think your flyer design is poor, you change the design to a perceived better version, and the response is again poor.
Deliver 5000 yourself (around 300 a go, every morning) and you know 100% that if the reponse is really poor then the flyer isnt up to the job.
Improve flyer, deliver another 5000, compare results. Hopefully the response is better.
Improve flyer again and deliver. Slowly the response should get better, untill you end up with a flyer that you know will produce the goods.
Then get this 'super flyer' delivered by a company. Then if the results are consistently poor you know there not getting delivered.
Test and evaluate constantly.
Daren