There are 3 basic components to any ad:
If your ad isn't working, it's one of those three items.
If postcards aren't working for you, it's not a media problem. It could be a market problem. Why? Postcards work and, if you test enough different versions of a postcard (changing the offer, mailing list, text, photos, look, feel, etc.), you'll find a winner. It's not 'if,' but 'when.' Sometimes, we get lucky out of the gate. Sometimes not. We keep at it, we'll win. If we throw in the towel, well...
I have recently found a "media" problem with FSIs.
And, it was confirmed not just with my own experience but that of two clients.
We tested an FSI in a paper in Philly. Over 100,000 FSIs dropped at different times, different businesses. One ad was a new patient dental ad. The other, a man's clinic ad. Both ads proved worthwhile in other markets and other newspapers. this paper however, ugly, ugly results.
So, when your ad fails, there's only 3 main places it's screwed up. Do some critical thinking before jumping to conclusions, too.