I have an idea that I believe will benefit two target markets (don't we all). I have tried reaching out to one of the markets (food truck owners) online (email & Facebook): 4 in total so far. None of them responded. Is it because there's something wrong with my message to them? How do you get to the right people to validate your ideas? Do you try all sorts of methods before you finally give up? Is the lack of replies an indication that my idea won't work, am I executing my outreach the wrong way or do I just need to try to reach more people?
I'd suggest polishing your in-person pitch and hitting venues where food trucks gather. These events happen weekly in major US cities. You should go one week to observe the customer lining up patterns and then pick an appropriate time the next week (maybe when the food trucks are wrapping up for the day) to approach the food trucks with your pitch.
Here's just one example of what I mean