If a simple circuit doesn't work (happens) it could be a faulty part (happens). Or something is installed with the wrong polarity (diode for example). Or a wire

If a simple circuit doesn't work (happens) it could be a faulty part (happens). Or something is installed with the wrong polarity (diode for example). Or a wire that should be conducting isn't. If you have spare parts, try them. If you don't, start building a junkbox. If you don't have an exact replacement, swapping in something reasonably similar will show you if that might be the bad part. Learn to test simple components for functionality with a multimeter (it's your BEST FRIEND). The book of experiments you did yourself is the best book to own. Expect some smoke (learning by fusion), and try adding stuff to see what happens. No book can replace the fun you'll have.