Regular PVC glue does not work when wet.
If you're using regular PVC glue to join PVC pipes and fittings together you must ensure the PVC pipe and fittings are dry and clean.
Then you put some primer on the pipe and fitting and then put the glue on the pipe and a little bit of PVC glue in the fitting as well.
Then quickly put the pipe and fitting together and hold it for a few seconds.
Then allow the pipe and fittings to sit for at least a few hours before turning on the water.
The glue actually melts the pipe and fitting together and does not actually glue the pipe together.
It melts the pipes and fittings together through a chemical reaction with the glue and PVC pipe.
Once bonded together the pipes and fittings should not come apart.
Also allow the water to flow through the pipes for about 10 to 20 minutes to clear out any glue taste from the water.