A loose plastic joiner is a small malfunction that can cause major problems if it's not fixed as soon as possible. In a home plumbing system, this might come with large consequences and cause major damage to your home. A loose plastic joiner or a plastic cap can cause leaks, which will redirect the water pressure and might cause the whole system to fail. Luckily, fixing a loose plastic joiner is not a difficult task, even if it's a part of a complicated plumbing system, and there are some things you can try to make it fit better before calling a professional plumber.
One of the most common reasons for ill-fitting plastic joiners is that debris or dirt gets caught in them. The first course of action when finding a loose plastic joiner in your pipe system should be to clean the joiner out. You can start with sanding the joiner as well as the connecting pipe to get all types of dirt and build up off. Use fine sandpaper so as not to damage the plastic, and roll it up tightly to fit it into the joiner. After sanding it, you should use a rag soaked with water to make sure you get out all dust particles created by the sanding. Avoid using detergents or other chemicals, as this might lead to more build-up when it has dried out, even if it cleans dirt out initially. It's hard to flush all remnants of detergent out afterwards.
You should also make sure to apply a solvent to your plastic joiner to make it fit well. Apply a thin layer of solvent on the joiner and the pipe. Only use solvents that clearly state on the can that they are made for use on plastic and that they are completely water resistant. Make sure you brush it on smoothly so that the solvent doesn't create uneven surfaces on the joiner, which might cause the joiner to fit even worse instead of helping solve the problem. If you do it correctly, the solvent will help fill out uneven parts of the joiner and make it fit better to the pipe.
After cleaning the joiner out and applying the solvent, you should mark out how you want the joiner to fit in the pipe. Use a marker to draw a mark on the joiner and the pipe. It will help you ensure they fit together perfectly. Also, make sure to use a protective insert when reconnecting the joiner to the pipe. This will keep you from damaging the pipe by pushing too hard. As plastic is a rather fragile material, it is prone to crack if you apply too much pressure when reconnecting the parts.
For more assistance or information, call a professional plumber.