The frustrating thing about brass pipe fittings is the moment when you doing an installation or making plumbing repairs when the connection leaks. You set everything right; you turn on the water and discover leakage. You go back, retighten the fitting or even restart making the connection and it still leaks. You do nothing but ask yourself what you are doing wrong.
There is a possibility that you are doing right, sometimes fittings are just low in quality or defective and you cannot do anything with them and throw away or replace with newer ones. And if the fittings are OK, you need to ensure that you are taking all the appropriate steps to achieve a water tight connection.
Threaded fittings, either metal or plastic, can leak if the fittings are defective or damaged by misthreading it on to its mate fitting. Once you apply some force to tighten the fitting that has been misthreaded, the female or male piece may be damaged beyond use. You need to thread the fittings with care and inspect them visually and use a sensitive touch to ensure the piece is threading appropriately prior using any force to tighten the fitting.
If the threaded fittings are correctly connected, the space between the threads is wide enough to allow water to flow. A fitting with a washer that is improperly seated may feel tight but the washer will let water around it and promote the leakage in joint.
Plastic pipe needs to be cut with square ends, pipe should be cleaned with correct solvent and all burrs should be eliminated. Once you clean the parts with the solvent, you should apply the glue. You can make use of applicator to swirl around the pipe two or three times and cover the whole stub that will go in the fitting. Insert the pipe quickly into the pipe fitting while providing a quarter to half turn. This will give you a sturdy, watertight joint.