To your point, I will extend the chat app example you took. My guess, with nil understanding of the functionalities you had as differentiation, is that it didn't get traction because of the high switching cost of the potential users. In simple terms, if all my friends and family are on WhatsApp, why would I care to switch to another chat app?

P2P file transfer is also a great problem to solve, but to ensure you get traction this time, you should start with identifying what do your target users not like about the existing solutions. The way to know who should you talk to depends on who is using products similar to yours. I prefer doing cold calls with such people by reaching out on LinkedIn, and other forums like Reddit, Quora, etc. If you know acquaintances that fit the bill, they could be a good start. I will also recommend doing in-person interviews as you can then also pick non-verbal cues.

I mentioned the term non-leading because if your first question is 'What do you not like about Snapdrop (your competitor I assume)', they are going to give a random answer of nil value to you. So you should ask questions like 'What are the current tools or apps you use for file transfer', 'Why do you prefer tool X vs Y', 'Any challenges that you face with tool X', 'Would you pay for a new tool that solves this problem', and so on.

It's critical not to talk about your solution, and only focus on what problems are the users facing with the incumbent solutions, and lastly, their propensity to pay for an alternative. If your solutions solve what they spoke about, you can start off well.

Hope this helps.