Well I do the same as you. And I get back from a client maybe 1 in 10 times (talking about when I am not recommended to a client, but just sending cold proposals on something they need).
In you case, let put yourself into client's perspective. If I want something and a guy tells me "yes I can do it for the budget and if you decide to work with me I will make detailed proposal". Would I contact this guy? Not at all. Or I would tell him to write a proposal without considering him seriously.
Now, back on the freelancer's side. Do you think you lost contractor because you wrote quality proposal? Don't be silly. You lost it because the client:
- was looking for cheapest work
- was looking for someone quickly without looking for quality
- was unserious client picking the first from the line probably the cheapest one again
- did not like your proposal or style
- posted a ghost job and never started to pursue it
So simply continue what you do and rely on your hunch. If you feel that short 1-liner will work, then make it such. If you feel that you need to make quality proposal, then make it such. I am sure with this approach, you will lose all unserious clients, but you are on a good path to get good clients.
As I said, I am doing the same way as you, and I have no plans to change my style. But I also rely on my hunch. So if I sense unserious client, I reply with short sentences trying not to waste time on them before I am sure that they are serious and that they have the budget for their project.