This is a stupid question but it has to be asked.Does your sales letter create as many sales as you would like. What proportion of them respond to your advert? What is just as important, how many of those that responded actually purchased your product?
How can you improve response rate? How many new email addresses did you capture? Do you have an 'opt in' strategy so that you can mail them later with more offers without being accused of spamming?
Let us look at sales letter first. The headline. Does it stand out? Does it grab your attention? Does it shout, "READ ME, READ ME, READ ME".
On other hand: Does it give a tantalising hint at something interesting inside. Something of real benefit to reader. Does it ask a question that can only be answered by reading content of letter?
The letter itself. Is it speaking to a group of people? Or is it a one to one conversation? How many times does it say 'You' compared with number of times it says 'Me, We or I'.
Your readers are not interested in you or what a marvellous gadget you are offering. No, they are only interested in what it can do for them. How much better off they will be if they invest their hard earned cash in your enterprise.
Will it help them to become rich, a better person, happier or better looking, Will it make them more attractive to opposite sex?
Convince them of just one of these, or similar things and price will not matter.
It all boils down to "What's in it for me."
The old adage: "Sell sizzle, not steak" is as relevant now as it was in old days of door knocking.
Let's go back to original sales letter. How good was it in first instance? Were you happy with initial response? if so, what went wrong? Has it become stale. Would a makeover put it right? Perhaps it just needs a new headline.
If click through rate is good, it would appear that headline is doing it's job. Should ratio of purchases to clicks-through be poor I would suggest that text of letter is not up to scratch.
The problem may not be in letter, or headline. It could be that you are not aiming your advertising at right people. Whatever you are selling, a very large percentage of population will not be interested in it. You must find those who are most interested in your particular product.