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There are several things that could be going wrong here so check them all to correct the individual or common problems.
I am going to assume that these are three part nipples.
The most likely thing is particles of some sort in the water and getting in between the two sealing surfaces. If the nipples were not installed with tapered end of the "plug" (uppermost part of the assembly) inserted into the nipple body first this problem could be especially aggravated.
Most likely cause is pipe that came to you with dirt inside. Dust from the storage yard. Ends jambed into the dirt when the pipes were handled. It might also have occured when you fabricated the system. What could have gotten in there? Maybe particles of the pipe itself when you cut it, especially if you used a saw.
Some medications in combination with other things in the water can cause granules to form and create all sorts of problems. If the system is old (I think not) there might even be mineral build up.
Did you hapen to use any substances when threading in the nipples? Lubricants come to mind. Some can change charachteristics and gum up the works when in contact with water.
Another thing to check is are the nipples in a very near verticile orientation?
