This page will attempt to address why a boat won't work. Assuming you followed the instructions exactly ("wet" and primed the engine), the problem is probably a leak. An engine with a leak will seem to want to start for a few seconds-- then nothing.
To test for leaks, fill one straw with water until it comes out the other. Seal your lips against the straws and blow into them hard to create pressure. Is water leaking out somewhere? You might have to look into a mirror to see exactly where it's coming out.
To seal a leak, clean the surface near the leak and thoroughly dry. Silicone will not stick right to a wet surface. Apply a layer of silicone about as thick as a coin.
You can't pressure test again for at least an hour, when the silicone is dry. However, when the silicone starts to "skin over" in about 10 or 15 minutes, you can do the reverse of a pressure test: a vacuum test. With no water in the engine, seal your lips on the straws again, but this time suck air in. You should be able to tell if air is leaking in without wrecking the repair. If anything, the partial vacuum might actually help to draw the soft silicone under the skin into the leak, making a better repair.
If there is no leak and it still doesn't work, prime the engine and try a couple more times. The only other thing that seems to cause problems is when the engine gets bent or crushed, or when a straw gets crushed shut.
Decades ago I learned how to repair my bicycle with a book. I'll never forget that before instructing the reader how to take the wobble out of a bike wheel with a spoke wrench--a particularly tricky task-- the book warned that you would ruin one wheel before getting the hang of it. It was right. I hopelessly bungled one wheel before I got it. Consider making another engine.
I am hoping to make an instructional video of making the putt putt boat because it is a lot easier to show people how to do it than to describe in words and still pictures.