I totally agree with Glennjanie. Pipe threads are tapered. Place a pipe nipple on a flat surface and you can see it. Pipe dope compounds are a lubricant, not a leak stopper. Plastic female fittings are prone to cracking. Many areas do not allow them in confined areas such as inside walls. If you still have access, look and see where the water is leaking from. Bet it is the elbow and not the actual threads.
if you have never made a mistake, you probably haven't done much.